Thank you, Saab, but it looks a bit nastier than that.
Thank you Conan, for your condolences - they are appreciated.
The drive is a standard 3.5" in a USB enclosure, so I'll transfer it to my hotplug cage and put it straight into the bus - taking your lead, DBTL - thank you. Looks like Ranish or some hard core fiddling to fix it, though.
I used the ultimate boot cd (well worth having) to mount some utilities and it would seem that the MBR is stuffed.
Weird - work is a bu99er at the moment and I'm able to stay focussed there, but at home I've turned into a newb. ISTR I do have a utility somewhere which fixed a similar problem, but don't have the concentration to find it again. When I do, I shall post the resolution here.
BOFH