and in the article, there absolutely zillions of articles...
BAILII is a free law site and you wil lbe able to find about 50% of the cases on it
the situation you refer to may well be covered by:
http://en.wikipedia.org/wiki/Bristol_and_West_Building_Society_v_Mothew
My guess and it is no more than that - is that the mutilated higher price is enforced and then you have to recover the difference from the agent
Otherwise - everyone would employ agents and then say oops I want to renegotiate....and that would allow them always to revisit a fixed contract
The other thing which is not a matter of law but is a matter of business is whether you wish to be saddled with a reputation for liking renegotiation.
You presumably have made sure the other party is not er making it up.
I like written notes for this sort of thing....
read Glazer's obituary in today Times and Telegraph