Spanners, I think you can't delete a "restore" cache whilst Restore is active so turn it off through:
Right Clik My Computer>left click Properties> click the Restore tab and check the box to turn off restore.
You should be able to manually delete the restore cache and then re-enable the Restore facility.
There is a way of doing this in one operation but I can't remember it at the mo...
However,
THIS may be some help.
fobs