Kill Claudio — P.M. Hubbard

While hunting in the English countryside, Ben Selby stumbles upon the murdered body of Peter Gaston, a friend of some twenty years earlier. As they had both been agents in a clandestine organisation, he is convinced that Peter was killed to prevent their meeting and sets out to question his widow.

What she tells him is the beginning of a perilous and exasperating search for something buried long ago on a remote island headland.