This is one of the Inspector Littlejohn mysteries. I figured it out early on in the tale, so I'd just call it a middling mystery, although I enjoyed the setting and found parts of it quite funny. It was better than, say, Murder in the Museum, but not so good as Death of a Lady.
The victim was odious, though, so having her drowned in a cess pool was satisfyingly ironic. I checked this one of out of the library, which was perfect. I'm unlikely to read it again, but I'm glad it read it in general.
I'm sure I will wedge it into Halloween bingo somewhere!