For the information of anyone else reading this, I did get the above idea working. But because there were some bad sectors on the HD, the 'dd' command failed on these and corrupted the filing system. Using 'dd_rescue' instead was OK although rather slower - it switches from 65536 byte blocks to 512-byte blocks when it sees the first bad sector but unfortunately then stays with 512 byte blocks for the rest of the copy.

Copying to the backup partition rather than to an image file makes it easy to verify the backup is correct and easy to restore individual files from inside Windows. I have not yet needed to do a complete restore, but the plan would be to do that just by reversing the dd command from the Knoppix CD, after restoring the MBR/partition-table by hand.

Peter