Murder Most Royal

I always include major spoilers (hidden), to help with my memory  issues. Read them at your peril! 

The royal family are at Sandringham for the queen's winter break, when a severed hand   is found washed up in a local beach. The queen recognises it, and despite trying to avoid involvement, she becomes involved again, more directly than in the previous two stories. The owner of the severed hand is a member  of a local aristocratic family, well known to the queen, the St Cyrs (pronounced sincere). It turns out that the motive for the murder was revenge for the rape of the murderer's fiancée several decades before