Originally Posted by Marsbar
It just occurred to me: would it be because the /boot folder isn't on a separate partition?
Well the short answer to that is, No.
The long answer is that the problem was actually because my hosting provider hadn't enabled the additional drive in the BIOS. So while Linux could see the drive (because it ignores the BIOS and goes straight to the hardware) grub could not.
Once enabled in the BIOS everything just worked (except I forgot to run grub properly once I wiped /dev/sda2 ready for the repartitioning and had to boot up using a Live CD to point /sda's MBR at /dev/sdb2 instead), so many thanks for the HOWTO, if it weren't for the engineer who put in my drive it would have worked flawlessly