Bumblescratch is a ‘ratty’ epic. 38 songs, mostly sung by Darren Day in a blonde beehive wig rescued from Mari Wilson’s dumpster. Kid co-star Ilan Galkoff is excellent as a sort of Stuart Little to Day’s Rat Thenardier and Michael Xavier gives the campest pirate since Robert de Niro in Stardust.

Otherwise, it proves that despite the modernisations of the format in Hamilton the prototype for a through-sung musical remains 1980s Les Misérables from which some songs seem ‘affectionately derived’, and that a clutch of Hogarthian chorus girls and a plague-ridden whore eaten live on stage by rodents make it a dubious outing for the kiddies.

This was a single Sunday night performance for a theatrical charity, but also professionally filmed to drum up investment for a full-scale production. So it would be unfair to review set, costumes, choreography or even performances since they didn’t have the same rehearsal time and production values as, say, the $11 million thrown at the Leicester opening of Finding Neverland.

Darren Day is a capable singer, and a popular showman. But his persistent adhesion to the spotlight is itself a problem. Firstly, you could only produce Bumblescratch professionally with a leading man who was also comedian, singer and major putter of bums on seats from his extensive fan base. Brian Conley was in the audience, possibly to size things up. But you’d never be able to cast it in a college or AmDram society – which is where writers derive a lot of royalties – since the rest of the company would kill whoever played Day’s part for hogging 75% of the show.

It’s written by Robbie Sherman, whose father and uncle wrote Mary Poppins, Chitty Chitty Bang Bang and a string of stuff for Disney.   This is a big and diligent piece of work but if a talent for composing were hereditary, Stella McCartney would be playing Wembley.

In the same way that Richard Rodgers’ grandson Adam Guettel didn’t make a better job of Floyd Collins, Sherman’s work feels studied and derived, and far too many of the songs sound like each other. This show doesn’t have a fraction of the inspired originality and variety of music and lyrics in Tim Minchin’s Matilda or Groundhog Day.

Although some rhymes need reconsidering, the lyrics to Bumblescratch are often ‘clever’ but it’s doubtful whether the intended audience would appreciate their complexities or keep children’s attention span through two and a half hours of a through-sung show.  Again, Matilda leavens the density of its lyrics with dialogue and comedy.

The narrative tracks the characters through the streets of 17th century London up to and including the Great Fire of 1666. King Rat Melbourne Bumblescratch adopts a young orphan rodent as his apprentice calling him Perry – ‘after the place in France’. Yes, quite. When he’s lying injured and in the first act closer Day sings something like ‘Perry, Perry … rat’ it sounds like a horrible accident in Nando’s.

In an abrupt and mawkishly-constructed scene, Perry falls for ‘Thamesa’ the daughter of a London baker who wails something not unlike Joanna’s ‘Green Finch and Linnet Bird’ from Sweeney Todd. Guess where the Great Fire of London starts.

This is still a workshop. The on-stage workload needs shifting from Melbourne Bumblescratch to ancillary characters who are better delineated, and Sherman could invite collaboration from a modernizing lyricist. However, it’s still a musical comedy rooted in the last century and whilst it’s a welcome original story rather than made from yet another movie – it doesn’t push the envelope enough.