I am apologising, as we take our first seats behind a big slab of a secret wooden door that must outweigh the All Black front row. It is a truly magic door, like something invented by Roald Dahl, just for children who drink – and lets us slip into a cool, smart hideaway in Soho; and I'm apologising to Bruce Robinson for not being one of this mag's usual stock of clever cheekboned women over-infused with vivre, panache, esprit and other words unacquaintanced in English.

But, then again, so is every female inside Mark Hix's establishment – not necessarily in words, but certainly with eyes and body language. It is astonishing how some people – this man is now 65 – still, somehow, just command flirty looks from waitresses 40 years the junior of him: a kind of courtly love prevails, even though he's in jeans and a dodgy cravat thing. He is, admittedly, charming, and effortlessly courteous, and still good-looking; but no one actually knows who he is, or if or why he's famous.

I do know who Bruce Robinson is, and am about to find out a lot more. Most of it in affably colourful language. It's safest for this magazine's sanity if I substitute the words "chuffing" and "todd" for the concomitant seven- and four-letter words Bruce quietly drops everywhere, through habit rather than guile or anger; fricative and plosive, they're actually right in almost all contexts. Anyway, Robinson is probably best known as writer/director of Withnail and I, but also has a fat backpack of screenwriting and directing successes (The Killing Fields, How to Get Ahead in Advertising) and an equally worthy history of falling out with Hollywood morons and "todds" and thus being perhaps less famous than he might be. He's about to accidentally remedy that.

The Rum Diary, his adaptation/screenplay/direction of the story by Hunter S Thompson is a delight; a marvel. Johnny Depp, who begged Robinson out of semi-retirement to write and direct, acts superbly and may now be forgiven for the dire continuance of the Pirates franchise. It's set in Puerto Rico, in 1960, and deals, in glorious primary colours, with journalism, racism, cold war politics, misogyny, Nixon, poverty and the flaws in the American Dream.

Bruce wants to move seats because he can't hear me; it's a lovely place but it is… clattery. And he has tinnitus. He charms teams of people into moving all our stuff downstairs to the quiet bar, and we talk about politics.

"I've always been that contemptible thing, a luxury communist. Sure, you can drive around in a chuffing Ferrari in my politics, provided somebody else isn't hungry. But all of this nonsense, Tory, Labour, Liberal, leaves me cold, what's the difference. It's a pejorative phrase, champagne socialist, and probably rightly – I can't drink champagne and I certainly hate chuffing socialism – but I also hate the idea of me being OK and some other chuffer being unable to feed their family. Now… shall we risk the oysters? Are you an actress?"

This last is asked not of me but of a svelte Swede waiting stoically for our order. Please, Bruce, I murmur afterwards, tell me you don't always say that to pretty waitresses.

"No, no, never, she sounded like a voice I'd heard on the BBC. Didn't know she was Swedish, she had perfect English. It's a bugger, being an actress, and we all have to help them. My daughter is one, and she's really talented, very and actually extraordinarily beautiful, but it's so hard, this industry. She's doing German clothes catalogues."

We talk a little about beauty: this man spent a good few years with Lesley-Anne Down.

"Eleven years we were together. Yes, she was rather a good-looking girl. Were you in love with her, too?"

Behind menu and blushes, I wonder if he has ever re-watched Withnail.

"I saw it, curiously, after about 15 years, with my son, who's 17. He'd been saying to me that all the boys at school were watching it. Bizarrely it hasn't aged. It's about trying to cope with generational change, which is universal, but it's also a film about anger. And, yes, I am so enraged by what's going on, with, say, bankers. That reminds me… there was a line of mine about humans being the only living things that claim belief in God, then act as if there was no God… rewatching Johnny doing that made me realise that's what I still feel about this, about bankers, about all of the todds on earth."

Ridiculously big question Bruce but when does that happen, when does the change or preference arrive?

"I think you're a good person or a todd by the age of eight."

Parents, is it?

"Well, I had the most appalling childhood. I was just chuffing hated as a child. Now, who's going to open the oysters? Oh, they're open already. Oh, man. They are delicious, aren't they? Doesn't this taste glorious, even more so when you remember eating badly? Those Camden years of mine, only option was saveloy, I almost starved."

I tell him about oysters I once had in Tasmania, at the Bay of Fires. Our guide, a certificated oyster diver, returned from the brine with a fat bag of them, shucked them on the shore and then pulled from the sea a roped and chilled bottle of Pipers Brook chardonnay. It was…

"No, no, NO!" he shouts, theatrically covering his ears. "Don't talk about heaven to me, because I don't believe in God."

He interrupts himself a minute later, as he talks about British restaurants. "You can remember when food was shit, can't you? It's happening in the cities, but it's not universal, and, oh, write down that line actually. I need that. 'Don't talk about heaven to me because I don't believe in God.' I love that line. I can't even think because I've just had a chuffing bottle of red."

Are you OK? I ask. He is clutching parts of his chest, has been giggling and the cravat thing has slid off. "Mmm? – absolutely. But I think I might soon be forced on to the pavement for a smoke. First, I'm just going to go and take my liver for a wash. See you outside?"

We have been for a fun cigarette and a cigar. Back downstairs, the mains await. "You've got a fish finger. You've got a chuffing fish finger!" guffaws Bruce, before happily sliding into his silver mullet and "Friday Chips"; actually, my Webster's Special is beyond delight.

"Everything we eat at home we grow, almost. We've got lambs up on the hills eating our wild flowers, and then we eat them. My wife does the cooking mainly; I'll do curries and stuff but it takes me all day, she's far better. We're in Herefordshire, best food in England. So I love food, but I also find myself obsessed with the chuffing world around me. Chuffing Iranians hanging homosexuals. And too many bits of America, though I love other bits.

Just before we part I mention that the film looked exuberant… fun… in the making. "Absolutely. American crew, and they were all saying the lines, laughing. We had that with Withnail, too. Gaffers and electricians, saying the lines. And no corporates, no box-ticking. If I'd tried the BBC today with Withnail, it wouldn't tick one box. They'd want every scene worked over with the question – why is that funny?"

On the stairs up I wonder whether he worried, much, about death. "I worry about death all chuffing day. Don't you? I'm with Keats. He wrote 'When I have fears that I may cease to be/Before my pen has glean'd my teeming brain…' – God, you need to read or re-read it… we all have so much to say, still."

The Rum Diary is scheduled for release on 11 November