Sayers' novel, adapted to a full-cast radio play, is a delightful mystery in the Columbo model, where our detective knows, from the get-out, who committed the murder, and the mystery lies in discovering how to prove it.

I've read quite a few of the Lord Peter Whimsey stories and feel like I know the character pretty well, but this audio drama did a lot to help me understand him even better, especially because the actor playing Whimsey knows how to deploy the Britishism "what?" to maximum effect.
The mystery itself is a fine example of the classical golden age detective story mystery. It's twisty, relies on arcane legal mumbo jumbo and British tradition, but follows all the rules one would expect of this kind of story.
At the same time, because the suspect makes almost no appearance in the story at all, it has an abstracted quality that reduces some of the impact of the mystery. In that way, Whimsey really embodies the armchair detective. There's even one point where he teases the policeman that he, Whimsey, comes up with elaborate theories of the crime and the policeman gets the tedious job of doing the legwork to prove them.

I've come to really enjoy the BBC radio drama versions of these stories--the life given the characters by the actors really works well for me, and they're light enough that I can listen while I work out.