I've bought and sold many times and my experiences of estate agents have never been good. I once found my own buyer, negotiated the price and did ALL the solicitor-chasing but, as they hade ONCE shown the guy round whilst I was at work, I had to pay them.
All they want is the sale and therefore the commission and sometimes, although they act for you and not the buyer, they tell potential buyers what they think will get them that sale. I have been told on many occasions by estate agents what they think I should offer but usually only after then property has been on the market a while and isn't getting any offers.
You may have a point about sending round buyers who are not prepared to pay the higher stamp level, but have you put it on the market at their valuation or your own? If they have told you they thinks its worth A and you put in on the market at B (i.e. more than they think) then they know they are going to struggle to get the sale at price A, and so will send buyers round who may offer less to tempt you to take a lower offer. You say that you haven't had much interest - so could it be overpriced?
In answer to your question I unless is actually written in the contract, they aren't in breach. If I were you I would serve notice now to end the contract. If they are past the 12 weeks, just get another agent. If you are still in contract then they know they've only got x weeks to sell the property. I served notice 10 weeks into a 12 week contract once and, lo and behold, they got their fingers out and it was sold before the contract expired.