Amazingly, if you are using Windows Server Backup, you have no way to delete old backup files. The service is supposed to simply grow your backup data until your drive is full, and then it should start cleaning up the old backups, but that isn’t ideal.
I found that I was able to delete the oldest backup by using the command:
DiskShadow.exe
followed by
delete shadows oldest f:
Where f: is the drive that is used for backups. There is no way to tell it to keepLatest 100 like you can if you were just deleting the system state, but you can put a bunch of delete commands into a text file and point diskshadow at it.
Chris May 
Hi
I have the same problem but as my backup drive is an external usb drive it doesn’t have a drive letter assinged. Do you know of a way to access and delete old backups from an external drive?
I have been looking around at various forums and can’t find a complete answer.
Paul
You can add a drive letter to the drive in disk management, and then give yourself permissions after if neccessary