A friend of a friend works for BA and was telling me that this happens from time to time. When she contacts customers to tell them this sort of thing, she has to rely on their goodwill/stupidity in order to get the rest of the money. Apparently, due to admin errors occasional flaws in the system have allowed people to fly to Thailand for �3!
You are not obliged to pay this as you have entered into a contract with the agent only, and agreed a sum which had presumably been agreed.
Dig your heels in on this one!