Apologies, The Dark Island was both a radio and TV serial - which the author, Robert Barr (himself a notable actor) then turned into a novel. The radio series (which post-dated the TV version) featured Edward de (not da) Souza, but I can confirm that the series was indeed "a taut, fun listen". More details here -

THE DARK ISLAND is story of espionage in the remote islands of the Outer Hebrides. It starts out with the discovery of a strange, unidentified torpedo, found washed up on shore of a small, nearly uninhabited island. It leads to a long-standing Soviet esponage ring
and an action-filled effort to find and capture the members of the ring.

It originally aired as a TV serial on BBC TV, starting on 8 July 1962. Later, it was done as a 6-part radio serial, airing on BBC Radio 4, from 19 September, 1969 to 24 October, 1969.

Both the TV and radio scripts were written by Robert Barr, who went on to write it into a
novel, published in March of 1973.

The radio serial is a taut, fun listen, very much based in the times, an interesting return to the late '60's Cold War times.