Firstly, I apologise because I'm aware there are lots of threads about this, i've read them. None of them quite match my situation. I've got myself in a right bl**dy mess!
I decided to try Ubuntu, and installed it on hdb, leaving Debian on hda. Didn't really like it (although it is astonishingly beautiful) so I got rid of it and made use of the drive.
I did anticipate this problem, I checked the /boot directory, but could only see references to Debian, so I thought it'd be okay.
There's not many commands available to me from this prompt, but:
./ ../ grub/ vmlinuz-2.6.26-2-686 config-2.6.26-2-686 System.map-2.6.26-2-686 i
./ ../ device.map stage1 stage2 e2fs_stage1_5 fat_stage1_5 jfs_stage1_5 minix_s
tage1_5 reiserfs_stage1_5 xfs_stage1_5 default menu.lst menu.lst~
I'm confident Debian is still in /dev/hda1. I'm sure it's simply a matter of telling grub not to bother looking for Ubuntu on /dev/hdb because it's long gone. Please help, because i'm not familiar with grub in the slightest.