It’s a cut-throat business, selling opera seats. Several major British opera companies have branched out into musical theatre as a way of doing so: ENO with its projected Sunset Boulevard, Grange Park with Fiddler on the Roof, and Welsh National Opera with this well-conceived and well-executed Sweeney Todd. The set is the day-room of a psychiatric ward (continuing WNO’s seasonal theme of madness), with steel containers that open to show Todd’s sparsely-furnished barber’s shop, Johanna’s chintzy bedroom, and Judge Turpin’s study, complete with prie-dieu and scourge. Mrs Lovett’s meat-pie stall is centre stage.

Diction can be a problem in musicals, even with amplification, and the opening chorus failed to spit the crucial words clearly across the footlights. Sondheim’s lyrics are complex, clever, witty and singable, but they need even greater clarity of diction now that audiences are getting used to surtitles even when the singing is in the audience’s language – or one of them, in the case of Cardiff. Todd himself was played by the German bass David Arnsperger, whose diction was the best of the cast, and whose physical presence was looming and threatening from the outset. Clearest of all was Janis Kelly as an irresistibly comical, sinister Mrs Lovett, who seemed to be channelling Joan Sutherland, Fanny Craddock and Mary Berry in rapid succession, permed blonde wig sprayed firmly into place.

Todd has returned from transportation to Australia in order to wreak vengeance on Judge Turpin for abusing his wife and stealing his daughter. Believing his wife has poisoned herself in despair, Todd wants to put Turpin to an equally terrible end. Turpin is attended by a bumptious beadle, who gets some of the best lines in the libretto, and who was played by the Welsh tenor Aled Hall, who exchanged his own accent for flawless cockney.

Todd fails to bring himself to kill Judge Turpin at his first opportunity, which they spend instead singing the sensual duet “Pretty women”, while Todd’s razor caresses Turpin’s jugular. Then Pirelli arrives on the scene, an ‘Italian’ barber (turning out to be as Liverpudlian as his performer, Paul Charles Clarke, a bit of luxury casting) who ends up half-dead in Todd’s banquette as the first ingredient for Mrs Lovett’s new range of meat pies.

Meanwhile, Anthony Hope (Jamie Muscato in winning form) is courting Todd’s daughter Johanna, against her guardian’s wishes, as the old judge wants, like Doctor Bartolo in The Barber of Seville, to marry his ward himself. Undeterred by the beadle’s brutality, he manages to get into Johanna’s bedroom and sing of his love for her. Act I concludes with the rapid – and on this occasion intelligible – patter between Todd and Lovett on the relative merits of priest, poet, Royal Marine, squire, grocer and vicar as pie ingredients, with delicious lines about “shepherd’s pie peppered with actual shepherd on top”.

By the second act, Johanna is in the madhouse with the other lunatics (the WNO Chorus method-acting for dear life), and Anthony sets out to rescue her. Mrs Lovett has recruited young Tobias Ragg to help her cope with the extra workload, and Todd has a brand new bright red barber’s chair, with a special lever that slides the victims down the chute and into the pie-shop kitchen. Customers for both shaves and pies are numerous, and the deaths get bloodier and bloodier, as Todd improves his razor technique.

The denouement is fierce and merciless, as Todd dispatches his enemy, the judge, and then the helpless beggar-woman, whose relationship to Todd turns out to be much closer than he had suspected. A bloodstained Tobias returns traumatised from the kitchen, where the oven looked suspiciously like the one last seen on the WNO stage in Hansel and Gretel – like the witch, Mrs Lovett goes up her own chimney in a puff of malodorous smoke.

The WNO orchestra played idiomatically and vigorously under James Holmes’s direction, and with the chorus in full cry and a splendid cast, the company made a persuasive case for Sweeney Todd to be judged as one of the best operas of the second half of the 20th century.