Brian Cox gives a mighty scowling performance as Bach in The Score at Theatre Royal Haymarket

Like listening to Bach’s music, familiarity and old-fashionedness ring through Oliver Cotton’s play about the meeting between Frederick the Great and Johann Sebastian the Even Greater. It’s the feeling we’ve heard it before. It doesn’t help that there was another big play about Bach just four years ago in London, Nina Raine’s Bach and Sons, nor that star attraction Brian Cox has actually played Bach before, in a 1984 TV movie called The Cantor of St Thomas’s.

That familiarity and old fashionedness aren’t a bad thing. It’s actually quite fun, and pretty unusual, seeing a proper old period piece, wigs and all, on the West End stage. But while the play has lofty ambitions and director Trevor Nunn knows how to stage them grandly, and despite a towering Cox as the main man, unlike Bach’s music too often the whole thing clunks and flounders.

Bach is tired. He’s old, he’s overworked and his eyesight’s going. Since Frederick the Great’s troops invaded his quiet hometown Leipzig he hasn’t been sleeping. There’s also an invitation on his kitchen table from Frederick himself to visit the royal court in Potsdam. It’s all rich terrain for Cox, reprising the role after the play’s premiere in Bath in 2023, to pounce upon.

Scowling, irritable, possibly dyspeptic, he does exactly that. As we’ve come to expect, it’s a mighty performance. There’s a sense of Cox taking Bach and moulding him into Cox: he’s a man full of principle and unafraid to speak his mind, even to a warmongering king. Like a squeeze box he inflates and deflates at will. Some speeches have him puffed up, rage in his eyes, bellowing about war and tyranny (the man can shout like few others). Moments later he crumples, tired and aged, especially in a tender scene which sees his son Carl (an earnest Jamie Wilkes) undressing him.

Surrounding these finer moments is a lot of chat and unnecessary noodling, like someone sitting at a piano and working out what to play: a snatch of a toccata here, a bit of a partita there. Cotton prods interestingly at the idea of creating art under tyranny, and how you cling to morals when you’re the patron of a bloodthirsty king.

But there’s just too much unnecessary padding. A servant in the palace gets a strangely prominent – and completely non-believable – role. Why have one toady composer who doesn’t believe in Bach’s abilities when you can have three? Fancy an unnecessary discursus on Hamlet? And what on earth is comedy French Voltaire doing there?.

It’s a play that could have done with sharp scissors before hitting the stage, certainly between its run in Bath and the West End. Take Cox’s wife, Nicole Ansari-Cox, highly-billed but underused as Mrs Bach, and one of only two female characters, both of whose bolted-on roles need either serious reworking or excising completely.

If the wigs didn’t do it, Robert Jones’s designs – austere simplicity in Bach’s humble home, gold and gilt in the Potsdam palace – would be a constant reminder that this is a big old play about big old stuff – music, art, beauty, justice etc – and it’s dutifully big and old and stuffy. And once the enjoyably crotchety showdown between Bach and Frederick is over, the piece runs out of puff, a slow fade out on a play which is all Bach and little bite.