I'm now able to use the PC in question. Using parition magic from XP, I see my reduced disk (130GB). When I run the Mandrake install CD, the partition screen shows the missing 30GB, as does partition magic when I boot from the rescue floppies I made before the first attempt at installing Linux (quite wisely for once!).
Not sure why windows and PM cannot see the unallocated space but it can from the rescue disk and install CD ... again, I'm not sure what they use to see the structure of the disk.
I'm a bit wary if running fixmbr in case it screws things up even more
When I try to create a new partition in PM from the rescue floppies (using the 30GB unallocated space) it only allows me to create an 'extended' partition without a drive letter which will enclose whatever type of partition I would like (eg NTFS/EXT2), and limits the size to 65MB, so I end up with a massive extended partition with a small NTFS one - no idea why this is the case and what this will leave me with!
I might try doing a fixmbr and see where it gets me.