A Spy's Ransom — Clive Egleton

Major John Tarrant works for the British Military Intelligence handling information filtered in from Russia and the Warsaw Pact countries. His life is peaceful until a Russian agent known as Drabble kidnaps his son and demands a ransom of 500,000 in uncut diamonds. Tarrant has seven days to come up with the ransom and less time to realize that his greatest enemy is not Drabble, it is a traitor of the highest rank in the British military