The above says the first delivery was in 1963. I believe that is the date which matters. Ideas and plans for computers are around for a long time before they come to fruition and can maybe be traced back for many years but the one date you can rely on is when a customer accepted a machine as working to its specification and paid for it. According to: http://www.chilton-computing.org.uk/acl/literature/news/1962.htm the Ferranti Atlas was working at Manchester University in 1962. I understand it had virtual memory. I wouldn't have wanted to pay for the quarter megawatt electricity bill !