It seems that everything is getting back to normal:
It wasn't quite as easy as I thought as I do not have a windows 10 installation disk - I upgraded from 8.1 online via an .iso file (which, I suppose I could have used to burn an installation disk!) However I had generated a Windows "10" repair disk.
The options I was given when, after making the D drive inactive, I booted from the repair disk, were not quite what I expected. For example there was no mention of Windows 10 rather references to windows 7 and 8.1, my previous O/Ss.
Fortunately I had done some research and had some instructions for tackling the problem in various ways.
What I did was to get into the command prompt and use the command: Bootrec /RebuildBcd
I then rebooted the PC and, much to my disgust, it would not boot from the C drive! So I reinserted the repair disk and, much to my surprise, on going to "Startup Repair" it then recognised that my o/s was Windows 10 and allowed me to boot from the C drive without any further problems.
The PC took about 6 hours to completely wipe the D drive and I am currently restoring all the data that I had backed up yesterday to it. I have every reason to believe that all will be ok when completed.
The problem with the repair disk may have been that it was generated with the system when the MBR was wrongly on the D drive, which had previously housed Windows 7 and Windows 8.1. In any event I shall generate a new Startup/Repair disk.
I am very grateful for the help received.
Kindest Regards
Jim