Have seen this loads of times and have sent people off to get the letter from council, now I no longer need to do this as the council have been putting it on the planning permission itself for the last while, obviously they were getting sick of issuing the letters too. Dont know which council you are with or how recent your planning is but our local council usually put it underneath the condition specifying residency as in part b or c of the condition, have a look at the planning in case it is there somewhere confirming that it doesnt apply in case of repossession.