No, you're right. C: hadn't been grabbed by the DVD drive, it was D:
I pulled the power or connectors to everything except the DVD and the new drive and started again. I then removed the 'other' partitions and did a quick format -- on the drive that was clearly marked C: on the DOS type blue screen. When I saw this screen, it reminded me that on the original install, C: was definitely the one that was selected for the system. But with a plethora of drives it must have jiggered the install, cos, now it has gone to C:
Thanks for the tip on removing the logical drives, now all I've got to do is reinstate them. Glass of wine first to improve clarity of thought.