The Dark Island by Robert Barr was broadcast on BBC Radio in 1969.
Story:
Spy Thriller: Set on an island in the Outer Hebrides in north west Scotland, a fisherman discovers what appears to be a torpedo washed up on a deserted beach. Upon closer examination, the container is found to contain materials for a spy (as the drama progresses, why the spy needs this stuff is never explained), and a couple of army officers go under cover to investigate.