I think I'll stick with my original plan though (fresh install on virgin HDD), as it serves two purposes...
1) It preserves Dad's system on the old disc, just in case I've overlooked any reason I might want to boot it up again some time in the future.
2) It allows me to make the only hardware upgrade I want, which is a bigger HDD.
Although it was my father's machine I can vouch for everything that happened to it, as every change involved a long and arduous phone call to the family helpdesk (muddle-desk?). I'm certain he'd not mind me saying so, would probably even raise a smile.
I was concerned that I may hit licencing issues reinstalling on a different HDD as some licensing software uses the drive serial number (among other things) to identify the licensed system, but I guess not in this case.
As for that Amazon Windows 8 Pro upgrade at under £50... no catches, then?