Is there a reason your landlord has claimed the deposit? Did you owe rent at the time you left?
I would suggest that you write to the landlord and demand payment with 14 days. If he does not pay, you will have to take him to court.
You can do this online at www.moneyclaim.gov.uk
I hope this answers your questions. If so, kindly rate the answer and provide feedback.
If you have any further questions, please do not hesitate to ask.