No operatic ending is more devastating, gripping and shocking than the final scene of Poulenc's Dialogues des Carmelites. Condemned to death for their faith by the French revolutionary court, the nuns walk to the guillotine singing the Salve Regina. One-by-one they are beheaded and the chorus loses a voice until just one is left, her words cut off mid-sentence as she, too, is killed.

The Royal Opera's revival of what I consider to be the greatest of all post-war operas has its problems, but that final scene is as heart-wrenching as it should be.

Sir Simon Rattle conducts a perfectly paced, nuanced interpretation of this wonderful score. And Deborah Polaski's Prioress is world-class.

But, alas, the rest of the Royal Opera's casting leaves much to be desired.

Unless the nuns' voices are easily differentiated, the glory of their music can too easily resemble a sort of tuneful mush.

And there is almost no variety between these nuns' voices. Put simply, they all sound the same.

Couple that with Robert Carsen's sparse production and it's very difficult to tell who is saying what on stage.

But that final scene compensates for previous shortcomings.