Thanks Gordon, I can follow that.
Unfortunately, I'd already managed to get myself into a pickle before your reply...
I copied the img.gz file over to the CM3 in Windows. Now after rebooting, the 4GB drive no longer shows in Windows, and so also can't be seen by the installer.
So I'm sorry, but two more questions:
How do I start afresh with the CM3?
After using rpiboot on the CM3, Windows prompts wanting to format the drive. Should I proceed or ignore?
Many thanks again, Roy.