Thanks for your input.
As I also had a problem with Time Machine - which would not run updates - it occurred to me that there was a more widespread problem in the operating system. I had run Apple's Disk Utility which had reported that everything was ok. I have now run Disk Doctor (which I should have done sooner) which came up with a lenghty list of broken links which it repaired.
TM is now working normally.
Had it not seemed that the problem was widespread I would have followed the route you suggested.