Lobbing a sweetener into his coffee in the posh Heights restaurant with its view over wet London rooftops, he looks slenderer than he has in recent photos. "One has to keep at the very peak of physical fitness to do what one does, and I don't like to have an ounce of superfluous flesh on me - it's corded muscle, the old corrugated-iron stomach. I'm not going to take my clothes off to show you - you'll have to take my word for it," he says, with the kind of slightly self-conscious drollery that ushers in each new day for regular listeners of Radio 2's Wake Up to Wogan.
It's hard to imagine anyone else getting away with Wake Up to Wogan. His trademark stream-of-consciousness blether has changed little over the past few decades. It takes a while to tune in mentally; the show is both surreal and sublimely silly. This is Wogan attempting to describe recent highlights: "Today we had Sumo wrestlers, and there was a lot of Japanese shouting going on right through the travel, and Paul Walters [one of Wogan's hapless team of "voices-off"] got hold of my belt and wouldn't let go. But I pushed him out of the ring in the end - we threw a lot of salt, it was messy. Last week the biggest reaction we had was to our firework display. It frightened an awful lot of animals, which is sad, so next year we're going to turn the sound down and just have the colours. And we have shadowplay - I do a duck, Paul Walters can do a rabbit. My head gets in the way of people in Southend - they write in and ask me to move it a bit. Apparently the view in Perthshire is very good, but then they're coming in over the top, you see."
Ummm. Yes. And when he aims this kind of thing into the airwaves, precisely what kind of listener does he have in mind? "A slightly hunched, unshaven figure in an anorak. We get tremendous fan-mail from homes for the bewildered, saying `Matron is allowing me to write this, but only in crayon.' There's a very high percentage of odd people who haven't got both oars in the water out there." Quite so, and it might be tempting to add auld Terry to the list if it were not for the magic ingredient that transforms what could be a complete farrago of nonsense: Wogan's voice, which is extraordinarily attractive - smooth and rich, a Bailey's Irish Cream.
The man himself, fresh from broadcasting, is all charm, resplendent in blazer with hanky in top pocket, grey-green eyes a-twinkle over ruddy cheeks, all mobile eyebrows and gentle chuckles. He is, he says, glad to be back in radio - he took eight years out between 1984 and 1993 to concentrate on television. Wogan, his chat show, went out three times a week for seven years; by the end he was fielding a fair amount of critical flak. "Anyone who does a talk show seems to get kicked to death. There is a confusion in this country on the distinction between a talk show and an interview. A talk show is about having a look at a famous face, a bit of stand-up comedy, knockabout stuff - an interview is what Barbara Walters or Connie Chung does in the States, in-depth, done properly. There was a lot of extremely unfair criticism, and also there was the perception that the show had failed, which really annoyed me, because it never did."
Despite his determinedly mild manner, the experience evidently stung. "Television criticism has become a kind of feature writing now, especially in the broadsheets, and it's tinged with hatred and envy and vitriol. It's using sledgehammers to kill butterflies ... Not that it matters."
He admits that not all his projects are top-quality telly. Picking up an award recently from the British Association of Songwriters, Composers and Authors, he observed somewhat acerbically that "a dachshund could present Auntie's Bloomers", (the tepid selections of BBC out-takes). Presumably it would have to be a specially intelligent, highly-trained dachshund? Apparently not; standards today, says Wogan, mean that small, yappy canines could easily take over most of the nation's broadcasting requirements. "Most television could be presented by a dachshund. Radio can't, although there are a lot of dachshunds in there. There's a lot of people doing radio now who shouldn't be let near it. They don't have the talent, they've been lucky, and they've got a bit of push. And there are people who are enormously successful in television, who in my view can't do it at all, who still behave as if they're on a stage - they haven't grasped the intimate nature of the medium."
After nearly three decades of loyalty to the BBC, he has declared that he is now "freelance" when it comes to television. "Presented programmes are thin on the ground these days. It's vets and animals, police shows and crashes," he says. Not that he is too worried. "I've never been the kind of person who seeks and knocks on doors - I've always been very lucky, I don't have any insecurity about the job." Apart from stage fright. "I've always had a low threshold of embarrassment - I'm probably completely unsuited to the work. I force myself to get out there. Audiences - well, they don't frighten me, on a good day - but it's not my favourite thing."
Wogan, now 58, has been married for 31 years, and has three grown-up children. His first child, a daughter, died shortly after birth. "It is very hard when that happens. One of the hardest times was when my wife Helen was carrying our second child - you worry. I was saying to Helen the other day that I would have liked another little daughter, or another little son, but it's only in your dotage that you start to say things like that," he says.
The family lives in Buckinghamshire. "I used to live in Berkshire, down by the river, but then a terrible thing happened - Michael Parkinson moved in just down the road, and I knew property prices would drop like a stone, so I sold up and did a runner. Shortly afterwards Rolf Harris moved in as well, so it was probably the best decision I'd ever made. There went the neighbourhood." Wogan-talk is larded with such gentle quips, delivered completely deadpan.
All the Wogan children are heading into showbusiness; Alan, 29, is a radio presenter, Mark, 26, a television chef, and Katherine, 24, hopes to be an actress. Though, says their father, being a well-known face is not much fun. "There's nothing to be said for being famous. It's a pain. You can't be rude to people, it's inexcusable not to be nice. Anyway, it's not in my nature. I was trained to be nice. For a period in the late Eighties, early Nineties, I was probably the best-known face in the country. But people do forget. Who talks of Alexander, who speaks of Napoleon now?"
Apart from his brood of fledgling broadcasters, he admires Chris Evans - a surprising choice, given that Evans is cheese to Wogan's chalk. "That's why I wouldn't make it as a presenter now. You have to be of your time. I'm not in-yer-face - I don't want to be in anyone's face, I could never be Chris Evans. But I've a lot of time for Chris. Maybe he needs to curtail himself, but his magic is in his manic approach."
Next Friday he presents BBC's Children In Need appeal. This, he says, makes his career worthwhile. "Everything else you do panders to the idle whim, it has no value. This has a value. So I'd like to continue till Hell freezes over - though eventually they will obviously say `Clear off, you old fool.' " In the meantime, on with the breakfast show. "I have a happy home, I like what I do and I have never had to worry too much about money. I have a happy temperament, a bit like Rebecca of Sunnybrook Farm." And, happy, he wanders off back to the studio.
8 Children In Need, Friday 22 November, BBC1, 7pm