Well, that's truly bizarre.
Plugged the laptop HDD into a spare desktop SATA port. It took about 3 hours for chkdsk to chunk through everything, and seemed to be doing an awful lot of deleting / reallocating / re-indexing….
… but at the end of it I could access the file structure and copy across the data I wanted to retain (mainly photos that hadn't been backed up elsewhere) to an archive drive.
Put drive back into the laptop, and whilst it wouldn't fully launch Windows, it would boot through to the repair console (where previously it wouldn't even read the HD / launch the OS)
SO, what happened?
I somewhat tongue in cheek blamed the Microsoft automatic update package – but seeing as there is no apparent hardware fault on the drive, what else could have caused such a catastrophic failure???
And, of course, MANY THANKS to the STW tech support collective 8)