OK - Found the problem, with some pointers from Beat
I trawled the history log and found the missing basket, which had been deleted by the client during their testing runs!
As a note of advice - I did check the history log previous to Beat's advice but I was searching for the user name. In this case the record shows the user as the admin, as they changed the record. If I had searched for basket id I would have solved my problem much sooner!
You live and learn.