Thanks big time to my mate Andy who found a DIMM (memory chip) had come loose from its mountings!
He's re-seated it and also given the machine a good owd tidy up, removing 3 years accumulation of dust. My only defence is I couldn't figure how to get the casing off the PC so couldn't clean it! Not only DIY I'm scared of!
(And the thought just occurred - why does "DIMM" seem somehow strangely appropriate?!)
Anyway - The Duchess bought him a big tin of Roses toffees as a "Thank You" because as I said - PC World wanted £50 just to have a look!
Sorting out back ups, restoring some settings and generally getting back into the swing of things so I'll try to update this nonsense more regularly in future.
More follows when I can think of summat else to say.