Here's the formula for impression based comedy. Rather similar to the topical comedy blueprint you take two news events apparently unconnected and incorporate them together in a bizarre scenario which demonstrates some hitherto unknown parallels between the two. Let's say you're a comedy writer and that the news items in front of you are: i) Parliament returns from summer recess. ii) The circus comes to town! Your comedic pay-off: Parliament is a circus! With a ringmaster, unruly animals and a paying public. You're in business.

"And now I mock your folly. And you become the fool. See what a powerful formula this is?"

Faced with a rapidly approaching deadline you might conjure up some joke about how in the circus they spin the plates, in Parliament they spin the truth. And then there's funny guy in the make-up and wig who nobody takes seriously. Or Mr Speaker as I call him. Ha! You thought I was talking about the clown didn't you? And now I mock your folly. And you become the fool. See what a powerful formula this is?

Dead Ringers certainly do as they rarely deviate from the blueprint. And they're quick to kick off their sixth season with an appalling Da Vinci Code take off Da Rolf Harris Code which was as baffling as it was unfunny. I'm not quite sure why Rolf Harris is topical unless it's the only impersonation Jon Culshaw knows people will recognise

"Seriously, is that the best they could come up with, that Jag rhymes with shag? A joke already used by a every sub-editor in the country for a fortnight?"

In other news John Prescott has admitted to an affair and David Blaine is spending a week underwater. Bizarre incorporation scenario: Prescott announces that to emulate David Blaine he is spending a year. In a pie. Because he's fat. See what they did there?

John Prescott gets it again and again and they pile on the pathos with the same trowel Rory Bremner uses to apply political comment. At the end of a GMTV interview Prescott bursts into song "I'm too sexy for my Jag. I'm desperate for a shag". Seriously, is that the best they could come up with, that Jag rhymes with shag? A joke already used by every sub-editor in the country for a fortnight? One that would struggle to get a laugh in a school playground?

But they don't just strike fearlessly at the heart of government. Noel Edmonds also gets it in the neck. They have him addressing the camera with "where the hell do I buy my shirts?". Criticising Edmonds' shirts - that's really going for the throat. OK, nobody said satire had to change the world but there's a general idea that it should be funny.

"'The only time they hit a target worthy of the effort is the 10 Years Younger spoof - accurately reflecting the shallow stupidity and cruelty of that ridiculous show.

Showing that they don't care whose toes they tread on they ruthlessly lampoon clueless indie filth Arctic Monkeys - pointing out that they're a bit like George Formby. If you say so, Jon.

The only time they hit a target worthy of the effort is the 10 Years Younger spoof - accurately reflecting the shallow stupidity and cruelty of that ridiculous show. But everything else misfires horribly. it isn't just bad writers or a lazy script editor that's the problem (although Dead Ringers undoubtedly has both of those) - it's the fundamental ethic of the show that is the problem. Each celebrity is surrounded by a marshmallow bodyguard wearing a T-shirt saying "nobody here gets hurt".

Dead Ringers is a cosy throwback to Seventies light entertainment where jokes about British Leyland workers and British Rail pork pies were delivered with such affection it was like a gentle ribbing around the Christmas dinner table from your family.

"This isn't take-no-prisoners comedy. Dead Ringers take prisoners, give them a comfy cell, call them a lawyer then release them without charge."

This isn't take-no-prisoners comedy. Dead Ringers take prisoners, give them a comfy cell, call them a lawyer then release them without charge. Comedy doesn't have to be nasty. Harry Hill has shown a finely tuned sense of absurdity can be brilliantly effective. But you've got to be very good to pull that off - and if you're not that good then you'd better start breaking some skulls. Because people came to see blood.

Of course, impressionists think that they don't have to be that funny. They think that the funny voice is good enough. So Dead Ringers is bedevilled by the same lazy set-ups and cheap lolly stick gags that have plagued every impressionist vehicle since Mike Yarwood.

It's this poverty of aspiration that means every impressions based show is stillborn. Until they raise the bar and start aiming for something that's funny without the George Bush voice that's not going to change.

The best thing about it: The 10 Years Younger skit.

The worst thing about it: The suffocating cosiness of it all

The verdict on Dead Ringers: Don't do drugs. Well, not that close to a tournament anyway.

Marks out of 10: 4