Hi Ivan,
Many thanks for replying and apologies for the delay.
It's a not a branded PC but one I've put together. It's behaved impeccably for years.
I happened to try most of your suggestions before I raised this query but did not fix the problem. I had uninstalled absolutely everything.
Anyway, I have now gone for the nuclear option and re-installed Windows from a USB stick since even Reset PC even failed. At least my PC is usable now.