Your deposit should be in one of the tenancy deposit protection (TDP) schemes. You can raise a dispute on their respective website and, if they agree that the landlord should give it back in full, they will refund your deposit themselves.

If it turns out that your landlord did not follow the law and your deposit isn’t protected, then you can take them to court. I would recommend calling Shelter and speaking to them before going down this road. And, as always, a strongly written letter explaining your willingness to take your landlord to court, citing your legal rights, is always a good first step before this point. Again, Shelter can help you with this.

https://www.gov.uk/tenancy-deposit-protection/disputes-and-problems