I think that your only option is to write to your ex-landlord and advise him that if the undisputed portion of the deposit is not propmtly returned, then you will have no option but to seek redress in the small claims court. This is quite easy: www.moneyclaim.gov.uk. With regard to your disputed bit, about we know nothing, there is tons and tons about deposits here and what is and is not allowable as a deduction.
Any information given in this post is based on my personal experience as a landlord, what I have learned from this and other boards and elsewhere. It is not to be relied on. Definitive advice is only available from a Solicitor or other appropriately qualified person.