Every landlord in the UK is required by law to register their deposits with the government deposit scheme (you should have received notification of this from the deposit scheme). This is called deposit protection. If a landlord fails to do this, they can be taken to court by the tenant. For failure to protect the deposit. The fine for this can be 3x the initial deposit. It would be advisable to let the landlord know this is your intended course of action should they not return your deposit. If a landlord has protected your deposit and they are of the opinion you require to contribute to any lost rent, damages to the dwelling etc. they require to raise this with the deposit scheme who will notify you of this intent. If you oppose this action, you can do so by reverting to them and advising of the same. If there is a dispute there is an arbitration department who can decide if, and how a deposit will be distributed or paid back. If they owe you any other money you can make small actions claim to obtain this back.
I shall provide you with a helpful link that will assist you in finding a solicitor/representative near your local area. This will provide you with someone nearby your area who can assist you if required.
I hope this information proved helpful. You will find a local solicitor who deals with these matters on the law society webpage which is as follows;
https://www.lawsoc-ni.org/solicitors (N. Ireland)
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