Eddy
According to disk manager and windows explorer my C Drive capacity has now increased from now 58.5GB to 149.95GB, leaving the 90GB extra from Drive D available. A good result. The result I wanted anyway.
The prompt on disk manager only gave the option to either reduce or delete "volume" on D, so I deleted the 5GB shown in the diagram to DavidR. As far as I know I did not delete the partition - just what was on it. Drive D no longer appears on disk manager though.
I has no idea what these files were, but I assume they are gone. (It is a second hand laptop I bought because I was not happy with Win 10 and wanted 7...so the previous owner may have put those there...hoping there was nothing system-critical in them).
Thanks everyone!