There are not many – any? – West End shows where you are greeted at the door by ushers in glittery cowboy hats who press badges and flags into your hands. Nor are there many shows where even before the curtain has been raised the auditorium is ringing to the sound of hundreds of clackers and the audience greets the cast with a Mexican wave and a suggestive Bosnian dance.

But then Eurobeat isn't strictly a show – it's a competition. On paper, the evening sounds far from promising – "a hilarious musical spoof of the Eurovision Song Contest", starring Les Dennis. As anyone who watched slack-jawed as Latvian pirates and knitting Bosnian brides took to the stage for the real thing in May might be moved to ask, how does one spoof an event which appears to slide further and further into self parody? The answer, in the case of Eurobeat, is with supremely silly and loopily enjoyable ease.

A hit at Edinburgh last year, this musical has a straightforward premise. Set in a Sarajevo television studio, it is simply a take-off of the real competition, down to the last detail. Even Terry Wogan makes an appearance. In the first half, 10 countries perform their songs. Then audience members – each of whom have been assigned a country with their welcome badge (mine was Sweden) – vote for their three favourites (excluding their assigned country) by text message.

After the interval, the real audience votes are reported back by the various countries' correspondents via authentically shaky satellite link-up (with, gloriously, occasional link-ups to the "green room" and the contestants' madcap backstage antics) and a winner is crowned in a shower of glitter and confusion.

The whole shebang is hosted by competitive limelight-seekers Sergei (Dennis) and Boyka (Mel Giedroyc). Dennis, in a Wogan-esque toupee and shiny suit, delivers the bizarre country fact-files in a convincingly nonplussed manner, giving proceedings just the right amount of seediness and innuendo.

But it is Giedroyc as Boyka, the former Olympic pole-vaulting champion, who steals the show with a procession of hideous frocks ("by Jeff Banks") and outlandish poses. Her superb rapid-fire tri-lingual delivery and eastern European mangling of English bring the house down.

As for the routines themselves, it would be unfair to give too much away. Suffice to say they incorporate all the classic competition elements from chronic overuse of billowing dry ice, to camp, lycra-clad formation dance (The KGBoyz from Russia, take a bow) and the classic Eurovision "reveal".

Craig Christie's and Andrew Patterson's songs are pitch-perfect in their ear-splitting Eurovision penchant for mixing unlikely musical styles (Italy's incorporates opera, hip hop and the twist), overstated themes and lyrics that don't quite translate (my favourite being Hungary's "I just killed some chickens and ate their entrails..."). Particularly convincing was the UK's entry, "I love to love to love", a meaningless duet of vocal histrionics sung by "two beautiful chavs with talent to burn... at the stake".

Some of the routines are too silly and a good few of the jokes fall flat. And, just like the real thing, the voting goes on a bit too long (unlike the real thing, though, it's entirely unpredictable). But the energy levels rarely dip and I can't remember laughing this much (or ever making quite so much noise) in a theatre. My jaw was aching by the end of the second routine and by the time Ireland appeared with their hummable crowd-pleaser, I had joined the rest of the audience in a spontaneous bout of singalong flag-waving.

If this sounds like your idea of nightmare and you don't find Eurovision at all funny, don't go. There is no other way to enjoy this show than by throwing yourself headlong into the silly spirit of it – and it's impossible to resist. And on opening night, Ireland came out on top. Like I said, it's just like the real thing.

To 15 November(01628 822 982)