It was a revelation that took me by surprise when I met them on one of their rare visits to London from Los Angeles, where they first went in 1974 to do an American series of Porridge, and where they now live a couple of streets from each other.
"We directed a music video for the Traveling Wilburys, so both of us have directed Dylan once," says Clement. "It's a little-known facet of our career."
In the light of their new film, you might suppose them to be rock stars manques. It turns out that Ian really is one (as you might guess, from his jet-black, Dylanesque curls, sharp if dishevelled blue suit and quickfire, friendly enthusiasm) whereas Clement is a rock manager manque, as evidenced by his more languid approach, cheery, well fed, Tim Rice blond looks and far more casual attire. Both expats are tanned; both married Americans; neither looks remotely 59, as they both are. La Frenais is short and hirsute and speaks with a Geordie accent; Clement is tall, clean-shaven and very Home Counties. But although they look and sound like the North- South divide, they think alike.
"Musicians tend to be very tolerant and very funny as a race," says Clement. "I envy their ability to speak a language without words, and communicate with each other and have such fun. When we directed The Traveling Wilburys, they had guitars in their hands all day and they never stopped, er..." "...fiddling," says La Frenais helpfully.
They regularly finish and embellish each other's sentences, a by-product of working together for 36 years, and even now clocking up a five-and- a-half-day week in each other's company.
"Yes, fiddling," agrees Clement, "and you just saw the joy they got out of making music."
Still Crazy has the interplay of affection, emotion and comic one-liners that is their hallmark: the insights into men together, the reflections of what might have been, the unfulfilled dreams. The dialogue captures character in a few phrases, with gags that hold the times up to such perfect ridicule that they have you throwing your head back in laughter.
It's all there in any episode of The Likely Lads, Porridge or Auf Wiedersehen, Pet, all of which have been enjoying TV reruns, a tribute to the timelessness and sheer class of their vintage material. Now it's the turn of the rock band Strange Fruit, with their rivalries, alcoholism and thwarted dreams. Few can equal their studies of men together. But they don't seem to do women.
"It's a valid comment," says Clement. "We tried to make Karen [the former groupie turned manager, played with an ethereal allure by Juliet Aubrey] an important part, to balance the testosterone. We do write the male group well. It's obviously easier to do that, but you try to stretch yourself..."
"No, it is very male-oriented," interrupts La Frenais. It's a rare disagreement, so rare that Gay News once attempted an analysis of the pair, which still amuses them.
They were introduced by a friend in a pub in 1962. Clement was a trainee in the BBC's African Service and La Frenais a "trainee in nothing". They wrote a sketch for Clement to use on his director's course and Michael Peacock, the first controller of BBC2, asked whether they thought there was a series in it. Which was how The Likely Lads began.
The series made their reputation, although they were always a little uneasy about being classed as light entertainment. They wanted to write versions of what they had seen in black-and-white Sixties movies about the North. They still think of their work as dramas, and ticked me off when I referred to Auf Wiedersehen, Pet as a sitcom.
TV, however, holds little attraction now. La Frenais is disappointed by the tone of most present-day television. "From game shows to comedies. Taking the piss out of people. Facetious," he says.
And the demise of their early-Nineties series Full Stretch, after six low-ratings episodes, still rankles. "If Auf Wiedersehen, Pet had been made now," says La Frenais, "they would have cancelled that after six. It only got 7 or 8 million viewers."
They got the idea for Still Crazy following a conversation that Ian had with Alan Price about the reunion tour of The Animals. "He said that after five minutes in the dressing-room with that lot, he knew why they broke up in the first place."
Whereas a band experiences break-ups, rivalries and bitterness, scriptwriters, it seems, have no such problems. "Absolutely not," says Clement. "We have gone our separate ways from time to time, but it was a leave of absence. We're very grateful for the relationship because we both feel we're better than the sum of the two parts. And we're still having fun."
"Perhaps it's different from bands because there's only two of us," says La Frenais. "I don't know; maybe there was a terrible break-up between Peter and Gordon, Simon and Garfunkel..."
"Hang on," interrupts Clement, "Simon and Garfunkel did break up."
"It's easier to reconcile differences when there's two of you than when there's five or six of you," says La Frenais. "There are always tensions, mostly to do with aims and egos, desires and frustrations. Of course, we've both realised for a long time that most of what we want to do is shared; it's a common pursuit. It's been reinforced over the years and it's too late now to break up."
Twosomes don't always mesh. The real-life Likely Lads - James Bolam and Rodney Bewes - did not get on well.
"Ah, well, those are actors," explains Clement. "They're forced together, and from day one Bolam was always rejecting the idea of being part of a team. He didn't want to be Eric Morecambe to Rodney's Ernie Wise."
The new movie, directed by Brian Gibson, has a marvellous cast, including Timothy Spall, Jimmy Nail, Stephen Rea, Hans Matheson and an exceptional performance from Bill Nighy as a shambolic, insecure, recovering alcoholic.
"There are such layers in Bill's performance," says Clement. "And the bonus is that he really sings all the songs."
The pair's next project is also based among rock legends, this time mixing fact with fiction. They have written a screenplay based around the late Keith Moon, as seen through the eyes of his driver. But The Who's Roger Daltrey has objected. He has his own Moon project in preparation and believes that the Clement/La Frenais script is not dark enough.
There has been a confrontational meeting, and the pair told Daltrey that they're going ahead. It all seems, a little eerily, as if though it could have come straight out of Still Crazy.
`Still Crazy' opens on Friday