In reply to Garbhanach:
Thanks for this - I had read one of these links but not the other. In it was reference to the autofocus drive pin. I tried to examine the mounts to locate it and noticed that there was some play that allowed me to see both pins retract thus eliminating them as the problem. So I tried pulling the lens away from the body gently while I released the pins. The lens came off along with a small arc of plastic. The mount had broken and the piece had stopped it releasing. So I now have a recovered camera and a lens that I can either have repaired or replace. A point to note is that this lens has a plastic mount - all my other lenses have alloy mounts. Something to look out for in future purchases.