"God, you look more and more like your picture," says Ralph Steadman as he leads me through to the comfortable kitchen zone at the back of his sprawling Kent mansion. He's talking about a cartoon he drew over twenty years ago for a column I'd been commissioned to write. And he's dead right about its advancing similitude. More's the pity. In the drawing my naturally broad brow has been given an extra inch or so and my cheekbones have been extended. My shock of hair is infested with flying bats and a large bolt has been driven through my neck. Not much ambiguity there. Thanks a lot, Ralph. Every year I'm getting to look more and more like Frankenstein's monster.

Matters might have turned out better if it hadn't been for my partner at the time the drawing was made. When she saw Ralph's first effort, which he'd based on nothing more than a couple of old photographs, she'd thought it a bad likeness and suggested that I take it back so he could make the improvements which she reckoned would automatically be prompted by seeing me in the flesh. I made a few protesting noises about what Rembrandt might have done if anyone had gone back to him with a similar complaint but she was a strong woman, so I reluctantly went round to see Mr Steadman at his studio in Fulham and hesitantly explained the problem. "It just doesn't look like me," I whinged. Instead of telling me to run along, he ushered me in, stuck the existing portrait up on his easel, slapped the three bats in my hair, ran the bolt through my neck, handed the portrait back, and said, "See if she likes that any more."

I tell Ralph the whole story as we start walking round the studios at the back of his house. It makes him laugh out loud. I'm glad about that. Before I came down to Kent to see him I'd been warned by a mutual friend that he'd recently been badly depressed – something about falling out with one of his much – loved children. So when the tour was over, and we'd finally settled down at the kitchen table, I decided to play safe by beginning with the distant past. What sort of child was he? Rebellious? Non-conformist?

"Not at all. I went to church quite happily. My mother was Welsh but because my father was very English she sent me to the C of E. As a little kid I used to wonder about the difference. If it was all about the same God why couldn't I go to chapel instead of the church? And she'd say 'Because you're a choirboy in this church and you're not a choirboy in the chapel. You've been accepted here so don't rock the boat'."

I tried without much success to picture the scabrous, scurrilous, grown-up Ralph as a devout choirboy. Was your father religious?

"Not at all. I used to say to my mother, 'Why doesn't Dad go to church?" And she'd say, 'Don't ask about your father.' So I didn't. But I gradually got to know his feelings. He'd been in the 1914–18 war and he didn't have any respect for a God who'd put him in that dreadful place. He'd say, 'God didn't give me a very good time, did he? Or those poor devils who were dying around me.' But that was that. He'd shut up then. He wouldn't say another word. When I was older I'd try to get him to talk about it more. But he wouldn't. He just got on with things. That was the way he managed. And my mother would always say, 'Don't bother him. Don't bother your dad'."

Was this a sort of lower-middle class way of not complaining about one's lot, a way of keeping going?

"Yes. Things didn't turn out for either of them. Nothing turned out. My father wanted to be a surveyor. But he ended up as a commercial traveller. And my mother, bless her, wanted to be a teacher, but she became a shop girl in TJ Hughes in Liverpool because she had to get a job to support her family. She did it without complaining because she didn't want to be a bother. She didn't want to be a bother and she didn't want dad to be bothered. She was such a sweet lady. In TJ Hughes they ordered her never to sit down. Never. And that gave her terrible varicose veins for the rest of her life. They came down here to Maidstone together when my dad got really ill. He was ninety-two and they brought him here on a stretcher. Mother didn't realise what was happening. She just said, 'Come on Dad, buck up.' And she had a cup of tea at his lips when he died. And he just went. No bother."

We've reached the sad present far too quickly. Perhaps we should go back to Ralph as a goody goody schoolboy.

"I was just a good boy because I wanted to be a good boy. I did all the right things. I joined the scouts and became a troop leader. Dad hated that. He hated anything militaristic. But as usual my mother said 'Don't make a bother.' She was the arbiter. She absorbed all the rotten bits of life and tried to make everything all right."

But there must have been a moment when he did start to bother, when he found something to resent.

"I think the first doubt was about the Scoutmaster. He loved to play a game with the boys in the church hall which he called Hyde Park. It was a simple game but it could only be played in the dark. We had to creep by him and his job was to catch us as we passed. And that meant he could have a good grope in the dark. He was abusing us.

"Then there was a new headmaster at school. An absolute sadist. It was then that I felt authority being imposed on me for the first time in my life. Authority for its own sake. My mother and father treated me gently. Father gave me the odd clout, but without rancour, without going on hating. But this headmaster was a proper sadist. So horrible that I retreated from everything at school and escaped into aero-modelling. I started with kits but then I'd draw my own shapes and make them and try to fly them. I'd never drawn anything before in my life except leaves."

It sounded like the young boy's escape in Kes.

"Yes. That was a brilliant film. But I carried it on further. I actually got into the De Havilland Aircraft Company in Chester as an apprentice. I was going to make real planes and then fly them. And that's where I learned engineering drawing. Front elevation, side elevation. I still make straight lines and circles in my work and that's where it comes from. It was the first thing in my life that I'd ever been good at. I got 100 per cent in my exams at Wrexham Technical College. But I couldn't stand fucking factory life. All the games. People nicking your tea and putting stuff in it. So I went to see the youth employment officer and he got me a job as a trainee manager in the stockroom at Woolworth's. And that's where I met up again with my sadistic headmaster. I was sweeping the pavement outside Woolworths one day when he came along. And he said, 'Just look at you. You have made a complete mess of your life. You're a failure.' And he walked right through my sweepings. 'Look at this stuff,' he said, kicking it away."

He did have a point, though, didn't he? You were going nowhere fast.

"I was. I got the sack from Woolworth's for fighting with the under-manager in the stock room, and then went back to the youth employment officer. He said, 'You've really done it this time. This is your last chance. Look, here's a careers encyclopedia. Take it away for the weekend and come back on Monday and tell me what you want to be in life.' Well, I didn't get any further than the A's. I chose Advertising Art. And after I'd shown him a couple of my drawings he fixed me up with a company that did trade magazine advertising. Dry stuff. But then I saw an advert for Percy V Bradshaw's Press Arts School. 'You too can learn to draw and earn £££££s.' And they had a cartoon correspondence course with people like Illingworth as teachers. I did the course while I was on my National Service. I drew what was around me. Boots on top of a bed. Guys sitting around playing cards. Then I'd go to pubs and draw pictures for pints. That's when my auntie – not a real auntie – helped out. She had a civil service job in Whitehall. And my mother and father asked her if she knew anyone in the newspapers who might get me started. And she knew Conrad Frost, a feature editor at Kemsley Press. He asked me to come for an interview and bring some drawings. And my first cartoon appeared in 1956. It was about Nasser and the Suez Canal."

But wasn't there still one huge missing element from this brief biography? How on earth did he explain the distinctive voice that seemed to be present in even his early work? What were the origins of the enormous radical energy and satiric violence that he was to pour into thousands of cartoons for newspapers and periodicals, and into his books on Leonardo, God, and Freud?

Where did he find the sheer anger and the absolute bloodymindedness?

"Well, there was certainly no one in my house with what you'd call a 'voice'. The only critical voices in our home came from the radio on the shelf. Mother and father just got on with it. In fact my father positively mistrusted anyone with opinions. He always said that people who spoke on the radio were 'very large'. Isn't that a lovely expression? 'Oh he's being very large', he'd say about someone on the radio. 'Very large with my pay packet'."

So, he didn't get his voice from home? And he certainly didn't seem to get it from party politics. He never seemed to have a straightforward left-of-centre line like, say, Low?

"Oh, I couldn't stand him. A carthorse with TUC written on his arse. Couldn't stand Low. He was a real brown-noser with Beaverbrook. A pretend radical."

All right. Forget party politics. What about revolutionary factions. SWP or SLL or CP?

"I avoided them like the plague. Just like I've always avoided teams and clubs. The best way to be an anarchist is to be a loner."

But where did the anarchic outlook come from? Books? Other people?

"I think that it came out of looking at animals. At their instinctual order. And then contrasting that with our elevated view of ourselves, our educated view that we are obviously better than anything else in the animal kingdom, when we are only another organism. I want to make sure that nobody is allowed to run away with the idea that they are superior. I don't like anyone who claims to hold the moral high ground. They can't have it. They mustn't be allowed to have it. That's why I don't like hearing religious talk of any kind. Islamic talk. Catholic talk. The moral high ground they claim to occupy doesn't exist. They've manipulated it into existence and I find that abhorrent. Nietzche said, 'It is only as an aesthetic phenomenon that the existence and the world are permanently justified.' I never forget that. That's all there is to it. Don't expect any more from it. There is no God up there helping you. When I hear Thought for the Day and someone like Jonathan Sacks starts talking in a perfectly rational way, I think 'What a mind!' And then suddenly he moves on to God and it all falls away. Suddenly he's as big a fart as the rest of them. What an awful presumption on the part of human beings to think there is a God up there and he is thinking of you specifically, thinking about you personally. What a dumb notion. We are an organism like anything else alive. As long as I remember that I stay firm-footed."

And he has seemed to stay angry. I tell him that on the way down to Maidstone, I'd bought an Independent and there he was on the front page with a powerful drawing of the horrors of contemporary Iraq. It was as tough and vigorous and uncompromising as anything he'd done in the past.

"Oh, I'll never get any smoother. I'll get edgier and edgier. I'm not trying to do beautiful, elegant drawings any more. Oh no. I'm in a hurry. Picasso is a sort of role model for me. At the end of his life he was doing three or four paintings a day or he thought he wasn't living. Right into his eighties he was producing painting after painting, entire series of etchings. Hundreds of etchings. Old self-portraits, haggish women, beautiful voluptuous figures. And cavaliers. People said: 'Where are the cavaliers coming from?' He explained that he'd been watching Errol Flynn films on television. That's great. Really fucking great. Anything that's going on. Absorb it. Reconstitute it. Keep fucking going."

I realised that I'd once again failed to solve the Steadman contradiction: the contrast between the affable, matey, slightly sentimental man who sits down and has a drink with you as though life can be left to go its own sweet way, and the artist whose paintings and drawings overflow with the energy of someone who has something so urgent to say about the world that it can only be said in the racy, scrawling, scratching, swirling lines that have become his trademark.

It's his affable side that's on show when we sit down to lunch together with his funny, clever, hospitable wife, Anna. Like all proper northerners, he's a good story-teller. And I know how to get him going. "Tell me what happened when Hunter S Thompson came to visit you here." (Ralph, of course, famously illustrated Thompson's drug-crazed classic Fear and Loathing in Las Vegas).

"I took him to the local," said Ralph ruefully. "Got him in there and, of course, he wants his usual bourbon. I gently explain that they don't do bourbon in small country pubs in England. Not proper bourbon, anyway. Perhaps he'd like to try a malt. Or at least a sort of malt. A Glenfiddich. Okay, he says, he'll have that. With ice. The barman serves him a standard measure with two lumps of ice. 'More ice,' Hunter demands. Two more lumps go in. 'More ice,' says Hunter. Finally the glass is filled to the top with ice. There's a long pause. 'Now the whisky,' says Hunter. 'It's in there already,' says the barman. Hunter picks up the glass, holds it up to the light, and peers inside disbelievingly. 'More whisky,' he says. Another shot goes in. 'More whisky,' says Hunter. Only after six shots does he relax."

"You must have been glad to get out."

"That wasn't the end of it. A few minutes later Hunter leans across to the barman who's now firmly convinced that he has a madman on his hands – did I mention that Hunter was wearing shorts – and says, 'Excuse me'. Ralph perfectly imitates Thompson's low-down sinister drawl. 'D'you know anywhere around here I could get a gun?"'

When I look back down the long drive to the house to give Ralph a final wave, I'm pleased to see that he is still bobbing with laughter.