A deposit can be held for up to 12 months, and must be returned after that date. Final billing for heating, water etc may not have arrived and the owner may want to ensure all this is finalised before refunding. I guess working on the premiss that if there are any outstanding liabilities they come from the deposit, rather than the owner having to pay and try to reclaim from someone no longer resident.

Frustrating, and unhelpful but your wish to save a few rap's on an exchange fee is likely of little concern to the owner, especially if he has a new addition to think of and and he doe have 12 months.

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How do you know if a Frenchman has been in your garden.... your rubbish is gone, your dog is pregnant.... but at least your cabbages are pest free