Thanks vapilot2004...........
I took your advice and downloaded the HDD Diagnostic prog, and it reports back that all is sweetness and light! I've waited a day or two before replying to see if anything developed but in that time the machine hasn't uttered a single untoward noise. As stated above, everything - and I do mean EVERYTHING - is backed up by duplicate means just in case.
I am about to invoke the 'working well, leave it alone' clause!
Thanks for your advice,
N o t a