The Third Sin — Aline Templeton

Pleasure is the highest good: the Cyrenaics practised the principle until the death of one from an overdose and the apparent suicide of another. Sobered, the group went their several ways. One heads to Canada, another disappears and a third is believed to have committed suicide, at least until his body turns up two years later in the wreck of a car swept up on to the Solway mud flats.
DI Marjory Fleming finds the case on her own patch, obstructed by the unpleasant and resentful Inspector she has been asked to direct, and DC Hepburn and DS Macdonald, still at loggerheads, don't make it any easier.