OK after a bit more digging it seems that the tapes where being seen as write protected (not sure why yet as have to wait for the customer to check the tapes). The backup job was not targetted specifically at the tape drive and so it started trying to do B2D backups which failed when it filled up the remote PC or it was turned off. So my initial understanding of the problem was incorrect.
I have targetted the backup to the tape drive only and if the tapes are not write protected via the tab then I will reinstall the tape device drivers, etc.
It maybe that BackupExec is labelling the new tapes with the same names as retired media and it's getting confused, I need to do some more testing to work that one out. Thank you for the response so far, I will update when I have the solution or if I am still stuck.