I ran EASEUS and shrank the W7 partition to zero, deleting the contents and when I got in from the gardening it all seemed to have worked fine. I now have the two "original" partitions on my internal drive, the sizes add up to the total of what I had before (a little more, actually) although running EASEUS again shows a few MB outside either logical partition.
Now that I'm happy with the program I am going to do a defrag on logical C and leave the two partitions be! I will probably do my annual W XP reinstall after Christmas from the recovery DVD then, if that all works OK will use EASEUS again to delete the logical D and revert to one logical drive.
Thanks everyone for the help!