FWIW,
The PC which is/was running Mint successfully, recognised the HDD as 160Gb. (See above). With that OS running, I started Ubuntu on a 'Live CD' basis this evening and then went ahead with the full installation which was successful and now sees the drive in its entirety.
Oh, joy beyond all understanding!
But, and there's always a 'but'... the neighbour tried to get Ubuntu going on his XP laptop within Windows this afternoon. He phoned me to say that the installation had fallen over and system restore wouldn't work. A bit of digging about showed that he'd managed to partition the drive at some point but denied all knowledge of it.
I feel a re-install coming on although I might try to delete the partition before going down that route.
Said neighbour is an inveterate tinkerer, but while he has free IT support on tap I suppose nothing will teach him the error of his ways.
I wish I could get one of those scissor arms with a boxing glove on the end, beloved of slapstick comedians, so that a swift and dire 'reminder' could be administered whenever he starts to delve.
And deep breath...
N o t a
PS Thanks for your interest, Gentlemen.
ETA: XP System Restore seems to have been comprehensively buggad.
It won't work and it can't be disabled. Tried that on the basis that a re-boot sometimes works.