Getting rid of problem letting agent

Tenancy Deposits

I wonder if anyone out there could provide any tips or advice on my situation. my problem is with my letting agent. basically i want to get rid of them as they have provided a pretty poor service. my property is fully managed by the agent. My current tenants are good and would like to stay on in the property, I of course would also like to keep them on, their 12 month assured shorthold tenancy comes up for renewal in just over a months time. They haven't yet renewed their tenancy agreement. The lettings agreement says any tenancy renewal would incur a commission at the same rate as the initial commission. Ok I expect that as most agents will charge this fee. What I want to know is can they still charge me a fee for letting them go and for keeping the tenants they intially found? Could they take me to court if I refused to pay? I have gone through the Lettings Agreement and don't see any mention of what would happen in such a situation.