I've fixed it good now.
Here's what happened:
1) IBM is in machine
2) Copied IBM -> Maxtor with Partition Magic 7.0 (having to resize the partition on the IBM first)
3) Maxtor boots fine, IBM doesn't - blue screens on booting.
4) Tried to resize the partition but PM7 reported uncorrectable dirty shutdown errors
5) Booted from Maxtor with IBM in as well - ran Chkdsk /f (not that it did anything) and shut down
6) Now able to resize partition down 14MB using PM7
7) It now boots again from the IBM!
What this all means is that it pretty much confirms my suspicions that XP does something proprietary at the end of the partition/disk. Either that or for some reason XP detects the disk as a slightly different size than the BIOS/PM7 does and conflicts with it when it tries to boot. For information I'm just using a standard on-board IDE controller (VIA chipset).
If I could be bothered I'd blank the last 14MB of the HDD and extend the partition again to see if there's anything in there that was making it go do-lally. Makes you wonder what on earth trustworthy M$ are storing in there...?
[Additional: I'm almost certain there's stuff there at the end of the partition because the Maxtor was freshly formatted and set up with the partition going right to the end of the disk with no problems]