How are we supposed to react to Not the Messiah, Eric Idle’s mock-oratorio rendition of Monty Python’s spoof of religion, Life of Brian? Since Idle was joined on stage at last night’s one-off by Michael Palin, Terry Jones and Terry Gilliam, it’s tempting just to revel in the occasion. And since the night ended with Idle and a choir singing Always Look on the Bright Side — brilliant! — and Palin bursting out of a Roman outfit and into The Lumberjack Song, we got bang for our nostalgic buck.

But the show itself, in all honesty, is an elegant, extravagant dud. Idle and his musical partner John Du Prez triumphed with Spamalot, their asset-stripping of The Holy Grail, by reinventing it as a Broadway musical. Here, there is impressive, inventive music, beautifully performed by the BBC Concert Orchestra, that echoes everything from Handel to Bob Dylan.

But only those with postgraduate degrees on the film will be able to follow what happens to this luckless non-messiah. Even they will find that setting great comic routines to music, as Idle does with What Have the Romans Ever Done for Us?, fatally dilutes the anarchic spirit of Brian. The grand, glorious backing makes the ideas drag.

Line by line, Idle’s libretto is deft — if you ignore the who-are-these-people-and-what-are-they-doing? problem. An operatic sex scene makes the form fit the content, is brilliantly performed by Shannon Mercer and William Ferguson (as Brian), and gets the ovation it deserves. And what Python nut wouldn’t enjoy incidental pleasures such as Palin reprising his Pontius Pilate, Terry Jones singing as a Welsh miner, Terry Gilliam arriving in his tails to perform one line and cameos from Carol Cleveland and Neil Innes?

Brian gives Idle his premise and his audience, but really Idle’s heart is in musical theatre rather than rehashing 1970s satire. The best moments of the show — which was being filmed — are where it’s not trying too hard to be funny. Yes, The Final Song repeats the power-ballad gag of Spamalot’s The Song That Goes Like This. But at least it has a flavour of its own. When They Grow Up is almost affecting. Or could be, in a show where it fitted in.

Marvellous to be in the presence of such masters, such music, such skill. But you sit, impressed but unexcited, wondering whether it’s just you or whether this is a lot of trouble to go to for such a middling reward.

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