I'd say no you wouldn't have to give the money back because you are hoping to go to a higher grade position. If however you had changed your mind and returned to your grade 5 role or taking a lower grade job for whatever reason, then I think you probably would have to pay it back.As you say, it isn't your fault that you cannot move, in fact your situation is being replicated all over the country and I know of two recent cases whereby two Signallers had to wait for a year before they could move.
Thanks mate.