can you 'sack' your estate agent before contract ends?

we placed our property on with an estate agent about 10 weeks ago, however, we feel they have not marketed the property in the best way - despite specific requests from us they have neglected to mention things we feel would be good selling points and give the bare minimum details otherwise. They are big on self-promotion (for instance, their company logo occupies more space on the property details than the floor-plan of the property!) We had to ask them to put a 'for sale' sign up as they apparently do not offer this automatically - anyway, the list goes on but the contract we have ties us in for 20 weeks, however, as we are not happy with the service they have provided thus far, can we 'sack' them and approach another estate agent to market without having to pay any commission to the original?? - thanks x