Elizabeth's personal fortune has been the subject of speculation for many years. Jock Colville, who was her former private secretary and a director of her bank, Coutts, estimated her wealth in 1971 at £2 million (equivalent to about £25 million today). In 1993, Buckingham Palace called estimates of £100 million "grossly overstated". She inherited an estimated £70 million estate from her mother in 2002. The Sunday Times Rich List 2015 estimated her private wealth at £340 million, making her the 302nd richest person in the UK.

Answer this question, if possible (if impossible, reply "unanswerable"): In what year did Jock Colville become Elizabeth's private secretary?
unanswerable