Having just come back from the Nazi Documentation Centre at Nuremberg, I must be one of the few reviewers who went to Big Brother Blitzkrieg because of its fascination with Hitler rather than its send-up of the BB format.

The concept is gripping: after his rejections from Vienna’s art school and a botched suicide attempt, the Führer wakes up in a technicolour box of what must seem like zombies as a contestant in the Big Brother house. How does he react to social media, contemporary slang, television itself – rather under-explored here, which is a pity because Adolf was actually fascinated by visual imagery and even proposed a sort of cable TV network for Germany as early as 1936

Writers Hew Rous-Eyre and Max Elton surround him with stock characters but can’t decide if they’re spoofs of actual media personalities or not and if, as suggested in Elton’s interview in the Wanstead and Woodford Guardian, it’s a satirical swipe at politicians like Boris Johnson and Donald Trump, it’s a risibly feeble one.  Over-confident Lucy seems based on Katie Hopkins in Jenny Johns’ sharp portrayal and the awful strident self-gender-defining Charlie could be her adversary Jack Monroe if it weren’t for Hannah Douglas’s Ulster accent. If this were a more topical piece, there would be hair-pulling.

The others are more generic: there’s an excitable, up-for-it, totally BB-besotted rapper who is obviously a middle-class teenager called Michael but styled M-Cat and Kit Loyd’s is the neatest of the spoofs. There’s a token gay screamer and overly sincere mid-life mum called Rachel who in the clumsiest of plot points is Jewish. Hitler’s assumed revulsion is diluted by authorial fear of offending and any opportunity for a wittily scripted Trump-infused rant about race or sexuality is lost.

Perhaps it’s true that you can’t spoof something which is already a caricature, and the poverty and last-days desperation of the Channel 5 version of Big Brother have already been roundly derided: it’s hard to devise anything more fictionally far-fetched than a show featuring Liza Minnelli’s ex-husband with a face apparently melting under the lights and a vermilion cretin from TOWIE.

Equally, there’s nowhere to take Hitler after The Producers and The Great Dictator, so what could make a great television sketch has been stretched to a repetitive 75 minutes in which your attention will almost certainly drift. There’s only so many times you can not laugh at “Will Adolf please come to the Diary Room”.

Set and costumes by Allegra Fitzherbert are good, and in a sustained performance, Stephen Chance makes Adolf credible and human and there’s an added dimension in the strange black-shirted climax.

We know from The Producers that “the Fuhrer was a terrific dancer”. Pity they couldn’t make him a terrific comedian, too.