If you commenced your tenancy after April 2007, your deposit should have been paid into one of three schemes, Tenancy Deposit Solutions Ltd, The Tenancy Deposit Scheme and the Deposit Protection Service, you should have been notified which within 14 days of receipt of the deposit.
My experience of Landlords and agents is clearly different to some others as I think the delay is probably due to some form of incompetence rather than an intention to retain the deposit. If there is no dispute the deposit should be returned if there is a dispute there is a free service offered by the elected scheme to resolve the matter. Discover who is holding the deposit and demand it is repaid in the absence of a dispute.