Last year gave us a slew of terrific new shows (Everybody's Talking About Jamie, Romantics Anonymous, The Secret Diary Of Adrian Mole, The Grinning Man, Superhero) that suggested powerful creative forces at work in new British musical theatre. It would have been nice to report that 2018 kicks off in a similar vein with Leon Parris' musicalisation of comic strip Bananaman, premiering in a go-for-broke production by Mark Perry at Southwark Playhouse.

Alas, this jolly, knowingly tacky, overlong offering proves as exhausting as it is delightful. Intended as a family show suitable for age six and upwards, it often resembles a much cleaner, less bad taste version of The Toxic Avenger – also featuring a mild-mannered protagonist who morphs into a superhero when exposed to a certain substance, in this case, bananas. Like that piece it possesses a juvenile jokiness that becomes wearisome; however, where Toxic had moments of revoltingly funny outrageousness and characters – however weird – we came to care about, Bananaman is required to keep it child-friendly at all times with the result that the humour is relentlessly one-note, most principal roles are too sketchily drawn to be of real interest and the plot is simultaneously tedious and hard to follow.

The music matches the humour: it too is relentless. To be fair, it's frequently melodic but almost entirely lacking in character, as though Pariss has absorbed the scores of Little Shop Of Horrors, Bat Boy and Joseph, removed any subtlety and prescribed that everybody belts their faces off. At all times. Nearly two and a half hours of that is pretty hard to take. The cast all have terrific voices, which is just as well, but the sound design is so dreadful that harmonies and lyrics are almost impossible to decipher when more than two people are singing.

Likeable Mark Newnham provides what little heart the show has as Eric Wimp, secretly in love with Emma Ralston's big-voiced independent-woman-in-training, while Matthew McKenna has some moments of genuinely funny cluelessness as his potassium-fuelled, masked and caped alter ego, as feeble of brain as he is impressive of muscle. Marc Pickering comes close to stealing the show as his gloriously camp nemesis Dr Gloom.

Mike Leopold's two-tiered set cleverly evokes a cartoon strip come to life, and Grant Murphy's rough and ready choreography has some rousing sections.

Despite the undeniable energy and commitment of the entire cast, and a couple of witty lines, the material feels desperately thin, and lacking in originality and variety, to justify over two hours of playing time.

Bananaman the Musical runs at Southwark Playhouse until 20 January 2018.