When I did the restore (I've just run another to make sure of the details), the Mark Partition active box was greyed out; but on the disk map the partition is marked as Act, Pri.
I also ticked the Restore MBR and Restore MBR from image file, as well as Restore Disk Signature and Restore Disk Hidden Track. Will try other restores without these settings.
After the restore completed, I checked the Boot.ini - it looks good
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows Server 2003 for Small Business Server" /fastdetect /NoExecute=OptOut
After the reboot, (which failed again), I booted from an SBS2003 Install CD, and ran the recovery console.
Fixboot C: reported that the filesystem on the startup partition was unknown (attempted to detect) and that the boot sector was corrput. A new boot sector was succesfully written.
Fixbmr wrote a new master boot record to \Device\Harddisk0\Partition0
Reboot after that started loading Windows! Now I'm at the Found New Hardware wizard...
Thanks for your help. :-)