You've got to be careful what you say about Stephen Sondheim.

There is a San Francisco-based mafia that will come after you with miaows, hisses and claws unsheathed.

I know this, because I am not one of his fans. But now that I’ve seen this production of his grisly 1979 comedy – starring Michael Ball and Imelda Staunton – I have undergone a Pauline conversion. To use the vernacular: I get it.

Jonathan Kent’s production is quite simply an overwhelming beast of a show, sweeping aside all before it. Previously I’ve only seen smaller scale versions, but the story of the avenging barber of Fleet Street providing lethally close shaves for his customers benefits massively from being blown up bigger on the Festival Theatre stage.  

It also benefits from being taken both very seriously and very frivolously. In doing so, Kent finds the perfect tone to match the shrill brass, doomy drums and driving strings of the music with the knowing silliness of lyrics such as ‘coriander makes your gravy grander’.

Meanwhile, Anthony Ward’s infernal set provides a suitably dark gothic, grand-guignol dungeon of black metal and damp brick.

But his design is also the site of delightful invention – in particular with a barber’s chair that folds into a chute, dropping victims to the cellar.

Despite being very much an ensemble performance, it is also the Imelda and Michael show.

Together, they are an exquisite, poker-faced parody of Mr and Mrs Macbeth.

Staunton is simply hilarious as the pie shop owner who recycles corpses. Tiny next to Ball’s strapping Todd, her batty, busybody pastry cook is mad as a handbag.

Just as good is Ball himself. Almost unrecognisable from the bouncing Ball of yore (it’s obvious from his new, slimline frame that he’s not been eating all the pies), he is the psycho next door, and a much-improved version of Norman Bates – taller, stronger and in control of his temper.

Just as Sondheim prescribes, his skin is pale and his eyes are odd.

But the great thing is how Staunton softens him up, making him a magnetic monster who never loses sight of his mission.

It’s a bold move putting such a sinister show on the main stage at Chichester.

But the night I saw it even the old-timers were up on their pins at the end, waving sticks and slapping knobbly hands.

We haven’t, I suspect, heard the last of this