OK, so I've tried sfc and DISM and both said all OK.

Thanks for your response. I omitted to mention that I had already run chdsk. However, I ran it again and that did not fix the problem.
I would rather not have to roll back, however, realise that...

All searching on my Windows 10 (upgraded from Windows 7) is painfully slow and often does not return any results. This is particularly noticeable in Outlook. This is what I have done so far:
1. ...