If there is a dispute between you and the landlord or agent as to how much deposit should be returned, then you can use the Alternative Dispute Resolution (ADR) process operated by the tenancy deposit scheme.
They will look at the evidence provided by the tenants and landlord and then make a final decision on how much deposit is to be repaid to you.
If you or your landlord do not agree to use the ADR process, then the dispute must usually be resolved by the county court.