'Ang up yer medals, Del

There is a moment in Macbeth when the title character, maddened by a vision of terrible future antagonism, yells at his tormentors: "Why do you show me this? ... What! will the line stretch out to the crack of doom?"

Which, I must confess, rather neatly sums up my reaction to the vision of yet another Only Fools and Horses Christmas Special, wheeled out in its bath chair to prop up the festive schedules again.

And, unfortunately, the news is that, yes, the line really might stretch out to the crack of doom. The man behind Del Boy, Rodney and the Peckham posse, writer John Sullivan, has said that he won't stop producing Fools and Horses, just so long as the public is still interested. "I've actually written two new episodes," he admits. And that's before the festive one has even been shown.

This is news to make hearts sink into boots, even among fans of the wheeler-dealer comedy. Surely the secret of any success is knowing when to stop? Only Fools and Horses was supposed to finish in 1996. Its subsequent swansong was due in 2001. Then this year's Christmas special really, really would be the final episode, honest. But no - it still carries on.

Now, before we go any further, let's get some things straight. Only Fools and Horses has been a fantastic success. It is blessed with wonderful comic characters and has given us some of the classic British sitcom moments of recent times (and, actually, not so recent times - it goes back 21 years, remember). As people never tire of saying, wasn't the scene where Del fell through the bar like an axed fencepost brilliant? And what about the time he and Rodders dressed up as Batman and Robin?

Yes, they are great moments. But the fancy-dress episode was six years ago. And the pratfall at the bar dates all the way back to 1989.

More tellingly, who can remember anything special from last year's Christmas episode? In case you're wondering, it was the one in which the Trotters lost a fortune, returned from Monte Carlo and took part in a Who-Wants-To-Be-A-Millionaire?-style gameshow to try to win more money (but not even Rodders's two GCEs were enough to ensure success). No, there weren't any classic moments. But worse than that, it was bloated and self-indulgent.

We have now gone full circle with the Trotters' rags-to-riches-and-back-to-rags-again story. So, what has Sullivan come up with for this year's festive treat? Erm, it's a rags-to-riches-and-back-to-rags-again story. At the start of the episode we are again back in Peckham, where Trotters Independent Traders are once more looking for dodgy ways to make quick money. This time, though, Rodney is the managing director of the company (how long before Del grows a beard and becomes the old Uncle Albert character?) and the latest scheme involves a trip to France, where the gang gets landed with some unexpected cargo.

Why put us through it all again? Well, first, because, despite being about as much fun as Christmas morning without batteries, last year's OFAH pulled in a massive 20 million viewers. And, secondly, because the TV companies have nothing else. We have just had a great year for TV laughs, with Ricky Gervais of The Office and Peter Kay of Phoenix Nights jointly crowned the new kings of comedy, but can you ever see either of them making a Christmas Day centrepiece? It's just not their style. So the schedulers are left to wheel out the old favourites (French and Saunders, Who Wants to be a Millionaire?, Des O'Connor ... they're all there again).

But watching the comedy of Del Boy and Rodders grow slow and arthritic is no fun. We've known them at their best; let's keep it that way. With friends, relatives and even ourselves, there is no escape - we have to live with decline, decrepitude and drooping. But, hey, this is a sitcom we're talking about. It doesn't have to be like that. Del and Rodders can win the pools, jet off to a new life in the Bahamas, get rehoused to Corby or simply disappear. They'll be gone, but never forgotten - because people will always say, wasn't the scene where Del fell through the bar brilliant?