Sinclair was promoted to major on 21 October 1961 and to lieutenant colonel 30 June 1967. In 1969 he was appointed Commander Royal Engineers Near East Land Forces, based at the Sovereign Base Areas of Akrotiri and Dhekelia. Following the September 1969 Libyan coup d'état in which King Idris was deposed by Colonel Gaddafi, the MoD issued orders to abandon a base in the Libyan desert, which had been intended for use in training British forces. Sinclair argued that the buildings erected at the site were valuable and could be used at other British bases. He flew to the site with Air Marshal Denis Smallwood, Air Officer Commanding-in-Chief, Near East Air Force, and his subsequent report persuaded the MoD to authorise the recovery of 85 Twynham huts, 2 aircraft hangars and a number of storage sheds. Sinclair returned from Cyprus in 1971.