Hi Duncan,
If you have Win 7 on disk and can boot to it
AND
you have Windows XP on any other partition (eventually restored from backup)
you can add a boot entry to BCD for Windows XP by using
"Dual-boot Repair tool for Windows 10 (and earlier)".
There is a special option "Fix XP boot" - click it,
then select Window directory of Windows XP using browser dialog,
click OK.
Done.
(The procedure above will add a boot entry for XP in BCD and eventually fix Windows XP boot related files if necessary)
That would be all.
If you have other questions or there is something unclear don't hesitate to ask.
Cheers
Bo