Thanks for your input.
As I also had a problem in Aperture - I was not able to create a new Vault - 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.
Time Machine updates are now working properly.
Should have tried it sooner - my apologies.