Avtur, I see your point and agree entirely. This was what we thought would be the case and they would then look to put someone else in our house.
However I fear that the rules are different as when you sign for the excess rent house their is a statement that you have to agree to in that you will remain for 6 months. I assume that if we were posted to Scotland tomorrow all bets would be off and someone would have to accept that they may have to pay for the empty house or find someone to put in it.