Termination of Letting Agent's Services

Letting Agents

Dear Sir/Madam
I have a letting agent who manages two of my rented properties. Despite continuous attempts to work with him I am unhappy with his lack of response when issues arise and feel like he is just acting as a middle man, collecting a monthly fee without being prepared to do much in return.
I have a very good relationship with the tenants and would like to issue them with new tenancy agreements directly between them and myself. The current deposits are held with the DPS, therefore I wonder if I could set up new agreements and ask for new deposits retrospectively once the original ones have been returned. If necessary I would be prepared to forgo the deposit as the tenants have been with me for some time now. The agent has never provided me with an agreement between him and I for his services, also the fixed term of both tenancies have expired and are running as statutory periodic tenancies.
Can you please advise the best course of action.
Regards
Lui Renzo