Thanks Hugh, Dave & John,
Hugh, believe me, it doesn't matter if it sounds stupid - I'll try anything! Thanks again.
Dave, good advice - I'll be re-stringing it today and checking the tree and adding a lubricant.
John, I had a new nut fitted a year ago (because of a similar problem) and, while the short-term problem was resolved, it has now returned. I also changed the trem springs from 2 to 3 and that seemed at first to have made a dramatic improvement but, as before, the problem has returned.
I'm going to try to find time today to strip it down and check all nooks and crannies. I'll let you all know if I find anything but I'm going to swap the tuning pegs first and continue with a process of elimanation.
Grateful thanks to all who have helped.
Malc.