trouble with letting agent

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my letting agent was really good no extra costs or unnecessary bills aprt from their fees 10% of rental for last 5 years. have had new tenants come and go -no renewal fees etc.perfect till the time when i decided to manage on my own. the last tenant has been in for nearly 21/2 yrs -comign to 3 years in april 2010. i ask the agent that i would like to end my agreement with them and manage the property with the current tenant myself. all problems start here. agreement dated 10.12.2004 found and looked into says "Our fee in connection with this service is to be based on 10% plus vat of all the rent received throught the whole period of the tenancy and any further extensions or specified additional letting period(s)." no mention on how i can end the ocntract .no mention of any notice periods.
the current tenant is on a rolling contract which means that he can give me a months notice and i can give him 2 months notice to vacate and this will end my agreement with the agent for that tenant.

help and advise from the forum appreciated.