Whatever was causing it seems to have stopped - it's all working perfectly again now!!
Thanks for the sugggestion Nightrider. I thought it might be due to the fact that the internal battery seems to be going, the date's stuck on 14th August no matter how often I correct both it and the time, but that's obviously not the cause.
Strange about the battery, though, I only replaced it about six months ago - I feel another thread coming on...........
