The MB AIR on loan to Hege & Ada was reported as being unusable , and sure enough there were only 800MB free out of the 80GB drive!
To access my account I started in Safe Mode by holding Shift during the boot process. Once there, I started looking for things to delete. Ada's Movies folder held quite a few GB, so I moved that over to my Black WD drive for a while.
Finally. I made a new Admin account, checked it worked OK before de-authorizing my iTunes account on the AIR, deleting the Music folder contents.. that got rid of a LOT, then from the new account I deleted 'gfraser' leaving the Home folder's remaining contents for the time being.
Have completed the job by making a fresh Superduper backup to Black 1 :)