In reply to ablackett:
I suggest you read whatever document you signed first, there's sometimes a clause in there about how long you have to stay with an agent before you can move although I think in practice these clauses are often ignored and you'll be free to go to another agent. Its not rare to see a house appear on the market with one agent and switch to another quite soon afterwards.
In reality, most estate agent related things are open to negotiation (including the fee, did you negotiate on that or go with their standard rate?).
I'd go in and speak to them firmly but politely and tell them you want to leave them. They're unlikely to refuse, its too much hassle, although they may try and offer you concessions or similar to keep you.
Do some research before you choose another agent, go in posing as a buyer and see how they treat you and how professional they are, do they have a lot of properties of your type on their books etc? There are some good agents out there but they are rarely national chains!
Note: I have no connection with any estate agent but I've bought and sold a lot of houses.