FIFTY years ago on Monday, Britain changed its currency. Out went pounds, shillings and pence, which dated back to Roman times and worked on a strange system of 12, and in came decimal pounds and pence, which counted in tens. February 15, 1971, was, as The Northern Echo said on its front page in big letters, D Day. This explained why the cost of the paper changed overnight from 7d to 3p. The pre-decimal currency was based on the Roman coins of silver denarii and gold solidi. Twelve denarii were worth one solidi, 12d = 1s. And 240 denarii were minted from a block of silver called a libra which weighed a pound. Therefore 1l = 240d, and 20s = 1l.