thanks for all your help, it's being most helpful! Problem is now fixed, was resolved by a reboot with fsck. However the funny thing is that no errors were reported.
What I think has happened is that some users use Xwin32 to log into the machine, and I think a rogue KDE has gone nuts and claimed a heap of resources. What I was unaware of and explains a lot, is that tools like du will only report the space being used by closed files, so files that havn't had thier handle closed will not be included. This would account for the discrepency between du and df. Found that the tool 'lsof' was helpful in starting to track down what went wrong.
Again thanks to everyone, glad that is solved and i've got that 80GB of disk back!
Fingers crossed though that dosn't happen again! Although I know the solution now, and it forced me to getting around to backing up a heap of stuff