You can claim online through the Small Claims' Court.
It is simple process, cheap and you don't need solicitors.
http://www.hmcourts-service.gov.uk/infoabout/c laims/index.htm
I strongly advise you to send a letter by recorded delivery along these lines first:
I wrote to you on [INSERT DATE], making the original request for a refund of my deposit . As I have not heard from you, I am writing to inform you I intend to claim the full amount together with interest up to the date of judgment and court fees in the proceedings through the county court. I look forward for a full response to this letter within 14 days, otherwise I will commence court proceedings to reclaim my money.