What're the Odds? - Downs Crime Mysteries, #14

Former Chief Inspector "Sarge" Downs comes out of retirement to work as a private individual attempting to re-unite a young man from Germany with a man who disappeared more than twenty years earlier. The man had been traced to Cairns and the German had flown in to Cairns to find him, but had no success. The current head of the CIB in Cairns, Nat Johns was approached and passed the young German's issue on to Sarge, his former boss.
The disappearance of the man took place from his digs in Cambridge and Sarge enlists the aid of an old Scottish friend, Donald McPherson who is a police officer in Inverness in Scotland. Between the two of them they discover that the UK Home Office and the former Special Branch and MI5 may have secrets to hide.