No worries, I solved it thanks.
It appeared that for some reason the upgrade left refrences to a device used in RH9. I solved this by loggin in as root, and changing the device to /dev/input/mice (which looked to be about right!). Not sure if this is the correct solution but would appreciate someone telling me if I have done anything dangerous!
X Server does now start now, and the mouse seems to be working too.
Wahay! I solved a Linux problem!
NB: So are the "files" listed in the /dev folder aliases for the physical components that they are supposed to represent? Or are they like device driver files?