Can I just delete the oldest files to make a space?
Yes, you can access the drive and delete what ever you want.
Thank you for your quick reply.
I had an Apple advisor over the phone helping me sort out my over-full Mac lap top. He said “Don’t delete anything on the Passport Air Time Machine as it may cause a problem” . So he got me buying ANOTHER Passport Air 1TB!
(But I am not too fussed at the extra. I shall just pass it on to my husband.)