First of all, big apology for not coming back to this thread for a long time. The pc (desktop) seemed to settle down for a while and other things got priority instead. Though just recently it started playing up again very badly, with the usual variety of faults - BSODs, failing to boot, cursor suddenly stopping responding to mouse, screen freezing sometimes with a loud buzzing sound, etc, etc. Virtually unusable. Sometimes useable for a day or few hours, sometimes not even 5 mins. Switching off and leaving it for a while gave a better chance of it running, but once it started playing up that was it until left again for a while.
The usual sort or symptoms that normally preceeded yet another new SSD. Though once the on board graphics card failed about 18 months ago and was replaced with seperate pcie unit - rather confusingly it also gave very similar symptoms to the failing SSDs when it was on it's way out. I was very close to pulling the plug and getting something else, especially as I don't know a lot abouts PCs, the person who normally repairs my computer is having open heart surgery and I don't know anyone else reliable to look at it and fix.
Before I binned it (it's at least 10 years old) I thought I would have one roll of the dice and put the brand new Corsair VS350 psu in that's was still sitting sealed in it's box in a cupboard. Nothing to loose.
Changed the PSU over a week ago and PC has been working faultlessly ever since. Can't work out if it was the psu all along, but then why would I get a few months of working pc each time a new SSD was installed? Had the SSDs really failed after only months? Anyway, new psu = perfectly working pc, so I'm not going to try to figure it it any further. Sometimes being a clueless parts swapper, you get lucky.
Thanks again to all who offered help and advice - really appreciated.