Turned out that the update was straightforward, albeit rather slow.
Having been forewarned of the possible problem - thanks Dakeyras - I changed the BIOS boot priority to the Win7 disk, installed SP1, checked all OK, and finally switched the BIOS boot order back again.
Luckily grub was installed a couple of years ago, and uses the boot sector of another disk.
I had installed Win7 at the same time as a new 1Tb disk, and it was the only one connected at the time. So grub had never touched its boot sectors.
Cheers,
Peter