Further to the above I ran sfc /scannow but there's no change in the situation. But was it working properly I wonder. It kept asking for the windows disk and I popped in the only disk that came with the PC, the Mesh Recovery disk. That seemed to satisfy it and it continued, only to ask for it again, I clicked OK and it continued etc until it apparently completed its task. Only very occasionally was there a flash of the light on the DVD drive.
Tried disk management again but still it doesn't work.