The current invasion of the West End by the mega-musicals is one thing, but it is something else entirely when one of the stalwarts of serious subsidised theatre defects. Ah well, always look on the bright side of life.

Simon Russell Beale, who has already wowed Broadway audiences in this role, makes the perfect King Arthur in this deliriously daft musical "lovingly ripped off " from the film Monty Python And The Holy Grail. Camelot is, once more, the place to be.

Russell Beale, until now best known for his slew of award-winning classical roles at the National and the Donmar, slips effortlessly out of iambic pentameters and into woolly tights. Anyone who saw this former choral scholar in Candide will know that he has a beautiful voice and it's a treat to hear it ring out in a multi-tiered theatre.

It's lovely, too, to witness his spoton comic timing. Those looks of bewilderment he gives as his band of unlikely lads hack their way across plague-ridden merrie England are priceless. In short, he's having a killer rabbit of a time following in the coconut-shells-for-horses'-hooves of Tim Curry.

The new arrival appears to have galvanised the rest of Mike Nichols's fine cast as well, meaning that lines, and indeed entire scenes, that were merely funny before are uproarious now. It's hard to offer audience members more than that infectious, all-too-rare sense of actors genuinely having fun.

I certainly laughed more this time around, even if I wasn't sitting next to a German man who took great delight in bellowing out "fart" and "shrubbery". For there's no denying that Python fans will fare best with Eric Idle's script, which includes all the expected awkward Frenchmen and Knights who say "Ni".

Yet the master touch achieved by Idle, in tandem with songwriting partner John Du Prez, is to broaden the whole thing out into a satire of musical theatre itself.

Hannah Waddingham's velvet-voiced Lady of the Lake sings The Song That Goes Like This, a glorious send-up of Lloyd Webber-style balladry. And those pesky Knights are no longer content with some greenery. No, here they demand nothing less of Arthur than a musical in the West End of London itself.

There's barely space to commend Tom Goodman-Hill's masterful take on various roles, accents, costumes and dance steps. For Spamalot's grail has been firmly claimed by The Actor Who Ought To Go Like This More Often.

• Booking to July 22. Box Office: 0870 890 0142