IT has been quite a journey for Only An Excuse, which is about to air its 22nd show on BBC1 on Hogmanay.

The 30 minutes of slapstick and parody about Scottish football and much else has become a Scottish TV icon, survived various brickbats along the way, and been a labour of love for Phil Differ, the show's producer and principal writer.

Together with mimic, Jonathan Watson, Differ has devoted decades to capturing the foibles and absurdities of the Scottish scene. In that time men like Frank McAvennie, Denis Law, Graeme Souness, Chick Young, Sir Alex Ferguson and a host of others have been given the parodist's brush of lunacy.

What exactly is the nature of parody in Only An Excuse? "It's a heightened version of the truth, maybe overly focusing on some wee thing you see in someone's character," says Differ. "Obviously, it's not the truth itself. I guess we home in on some slight imperfection in someone, and you heighten it. You kind of make fun of their world."

Even in solemn seriousness Scottish football can be amusing. Indeed, solemnity is the root of the whole Only An Excuse canon. In 1985 William McIlvanney narrated the brilliant but slightly melancholic Only A Game, the TV paeon to Scottish football, and McIlvanney's lugubrious voice and delivery immediately appealed to Differ for some sort of take-off.

"Only A Game was wonderful but it lent itself to being sent up," he says. "McIlvanney is a poet, and he had lines like [sombre voice] 'the bleached bones of the Scottish game' and so forth on that programme. As parodists, it gave us a template from which to work, especially McIlvanney's voice.

"Back then I went to see Colin Gilbert [of the BBC's in-house comedy unit] and said: 'I've got an idea that might make a funny parody.' The name came out of the air like a flaming arrow...we would call it Only An Excuse, the 'excuse' being to wallow in self-pity and moan about Scottish football.

"We did it originally as a pilot for Radio Scotland in 1986. Nobody had ever heard of anything like it at the time - a comedy show totally about football - and it caused a ripple. We were chuffed."

Back then in the mid-80s Hogmanay staples like Scotch and Wry and IM Jolly were still holding sway, but Only An Excuse quickly came up on the rails and took over the old year night's prime slot in 1993. The secret of its success, says Differ, has not been hard to glean.

"It's the nature of Scottish football, isn't it? We just take it far too seriously. I'm sure there are people in Brazil who take their football seriously as well, but we can be parochial and deadly serious about it all.

"In this country we get football on the front page and the back page. Or quite often you can turn on the TV news and the second item can be a football story that many would view as utterly trivial."

Some of the show's parodies are funny and perceptive, but I asked Differ if he felt others over the years had bordered on a bit cruel. Denis Law, for instance, as brilliantly as Jonny Watson did him on the show, was depicted to some as an imbecile. But Differ disagrees.

"No, I think in Denis's case we make him spaced-out and almost stream-of-consciousness. Jonny has met Denis umpteen times and there has never been a problem.

"Down the years I honestly can't think of anyone who has taken real offence at us. Our rule has always been, if we are in a room, Jonny and I, and a guy we have taken the piss out of walks in, then it's okay for us to feel a wee bit uncomfortable. But if we feel really bad about something, then that means we've gone too far. And we've never, ever felt that.

"Some characters have even given us advice on how to do them better. Alex McLeish, who has a squashed nose, said to us, 'if you are shooting me, always do it from this angle, because that's how I always do it on the telly...from the other angle my nose looks a lot worse.' We even have a make-up artist who can paint an 'indent' on Jonny's nose when he's doing Alex."

Watson, whom Differ describes as "a brave actor", is a fine impressionist, but both creators have admitted that some characters have remained beyond their reach.

"There are some people that, for whatever reason, the writer and the mimic just cannot get. Going back years, Billy McNeill was one we just could not get. And Jonny - fair play to him - tried and tried but for whatever reason he just could not get Ally McCoist. You'd think it was easy but, physically, Jonny just couldn't get into the character. Ally says things like 'shooperb' but if an actor is not comfy doing the impression, then it can't be carried off."

After all these years, Only An Excuse, like most other things, has not been immune to criticism. The show has been lauded, though others have said they tired of its jokes and format. Thus far, the ratings keep it happily bubbling along, but Differ is aware of its shelf-life.

"The trouble with having a show that has been going a long time is that some people will always say, 'aye, but it's not as good as it used to be.' Or some will say, 'why do you still have Denis Law in it?' when in fact Denis Law has not been in it for about seven or eight years.

"That is the perception of it. So what you do is, you try to refresh the cast every so often. I mean, the show is built around Jonny ... Jonny is the given as an actor and mimic. But we bring in other actors, other subjects - like politics - and try to mix it up.

"This year we've changed it again. We've broadened the scope, with a lot of stuff about politics, the Commonwealth Games, and crazies like the ice-bucket challenge. So we've opened it up further, but still with that football ethos."

Parody is parody. Frank McAvennie insists he has never once in his life uttered either "shockerooni" or "whaur's the burdz?" But Differ puts me right on this.

"I don't think Frank has ever said 'shockerooni'; that's true. But he has said 'whaur's the burdz?' I know this, because Frank once had a pub in the Gallowgate, where one day he was due to interview girls for some barmaid jobs.

"A pal of mine, Andy, was in there and said that Frank duly showed up, looked around for his prospective interviewees, and said: 'Whaur's the burdz?' So I know for a fact he has actually said it."