Two writers of musical theatre are trying to pen a new show in this musical. To add to the meta mind scramble, they seem to be loosely based on Toby Marlow and Lucy Moss, the creators of Why Am I So Single?

“Write what you know,” suggests their agent. That truism did not hold for Marlow and Moss’s stratospherically successful first collaboration when they were still at university. Their fantastically subversive and uplifting Tudor musical Six, featuring Henry VIII’s wives as rock stars reincarnate, remains a West End sensation.

These characters, 20-something best friends entirely of our time, follow their agent’s advice and convene on a sofa to figure out why they are forever single. The concept brings so much knowingness that the drama does not quite enter into any real storytelling territory for some time, stalled by the construction of itself: characters name – or rather disguise – themselves as Oliver (Jo Foster) and Nancy (Leesa Tulley) after their favourite musical. The playful theatricality and cute visual punning are reminiscent of Emma Rice: the furniture on Moi Tran’s set design is animated, so an actor in fancy dress is a lamp while another, in green, stands in for a plant.

Co-directed by Moss and Ellen Kane, there are skits on unreliable men who cancel dates, and those who play power games on texts or lie about their height online. The barrage of “trashy men” these two encounter is embodied by an actor dressed as a dustbin. It is all amusing though rather familiar and fleeting, the scenes not quite joining up with each other.

Beneath the japes, potent issues bubble, from Nancy’s break-up with her ex to Oliver’s trauma around identity – his/their pronouns shift, and there is an undertold backstory of hostility and surreptitiousness around sexuality, with a momentarily searing song in Shhh!!!!. It is performed with smiling anguish as a Marilyn Monroe-style number but the inner drama of both characters is too quickly submerged.

What elevates the production is the score: every song is a powerhouse, even a preposterous one about a bee in the flat (Interlude in B Minor). The music ranges across genres, from rap to disco and a fabulous tap number in C U Never, to rock and musical ballad. It is possibly not as infectious as Six but the songs are better crafted, more heartfelt, the lyrics gleaming with wit, intelligence, bravura and, in the second half, emotional heft.

The central performances are strong, too: Tulley fills her voice with feeling though Foster has more depth to their character. For tone and story, you wish for a trajectory with less archness and greater momentum. Or for a more cohesively arch play that leads to something beyond routine summaries of the dating game.

The show resembles Michael R Jackson’s searing A Strange Loop, in its self-referential central premise but it worked to bigger and deeper effect in that musical. This really does feel like two people on a stage with only one other recurring character, Artie (Noah Thomas), and an ensemble that never really comes to life outside Kane’s fun, pop video choreography.

The power of the drama hits in the second half as the characters become more vulnerable and intimate, with immensely moving numbers in Lost and Better Off Love Story. You see what the musical could have been, if these issues had been better integrated and extended. With material this alive – about friendship, family and heartache, all unrelated to the search for men – the first half about dating woes feels old hat as Bridget Jones, in spirit.

The songs remain brilliant though, and in some senses the story seems secondary. Moss and Marlow are without doubt the most talented musical songwriters out there. I look forward to a long and illustrious partnership now that this difficult second album is out of the way and they are released from the burden of having to write what they know. Long live their friendship.