As you suggested, I downloaded and installed OnyX from the VersionTracker website.
Once all the initial verifications had been completed, I located the 'Rebuild LaunchServices' option and ran it.
A couple of minutes later upon completion, I was able to do the 'Software Update...' and 'About This Mac' as normal
Thank you very much for you help... my confidence has been restored
Also impressed by the very rapid response.
Kind regards
John