iCloud Back Up Won't Delete

I have recently been given a new phone (6+) by Apple due to hardware failure. I believe the girl helping me did something with the back up on the old device which stops the new device backing up onto iCloud due to lack of space. I am now not able to delete this old back up from my iCloud account. Every time I try to do so I get this message;

"this backup cannot be deleted because it is being used to restore another device"

I haven't got this other device anymore! So not sure what to do. This was around 12 hours ago. I really could use the space to back up again - can anyone suggest a way to get rid of this back up? perhaps with terminal? or the like?

Thanks!