Hi Sup3rsprt and Saltgrass,
I couldn't use the Disk Options because I got the error message before getting to that screen.
I still had Vista and a Windows 7 RC installation working on 2 partitions. I had Paragon Disk Manager trial installed on the Windows 7 RC partition. I used Paragon to delete the 'second' of the 2 'invalid' partitions. It then became possible to assign a disk letter to the other invalid partition. I guess that must have been the 'invalid' Windows 7 RC that I had been able to run.
Anyway, I could now do a custom Windows 7 install without getting the error!!
So, WIndows 7 is now installed. I'm taking it slowly, making disk images every step along the way. Paranoid? - possibly.
Thanks for your swift responses,