Agent puts tenants in without notification

Tenancy Types and Management

I use a managing agent to rent one property to four tenants. The initial contract is due to end in November. Recently I recieved a new contract from the agents replacing two of the tenants, the new contract runs to September 2010 locking in the manager's fee for another year. The agents did not notify me of the changes, the negotiations or ask me whether I wanted to extend. The two new tenants moved in before the start of the proposed new contract and started paying rent through the agent. I feel this has taken away my ability to decide what to do with my property and forced me into paying the agent for another year. Please could you let me know if I am in a good position to be able to end the relationship with the agent but keep the tenants?

Al