Good Lawyer Guide's response

A landlord takes a deposit, usually of a month’s rent, as a kind of insurance policy. If a tenant does not comply with the obligations to keep the property clean and in good condition, the landlord will use the deposit as a reserve fund to pay for any necessary cleaning and remedial work.

Typical issues which may cost a tenant his deposit include damage to furniture which goes beyond normal wear and tear, rubbish removal, replacement locks and keys, and so on. Do make sure your rental agreement sets out what can be deducted from this deposit and in what circumstances so that you know exactly what it could cover. Make sure you keep the flat in good condition and there should be no reason why you won’t get your deposit back when you leave.