Just to be clear, you have no legal basis to say you don't have to pay. If it's in the contract, let's just assume for the sake of argument this thing goes all the way to the Court of Appeal, my money is on 9 times out of 10 you are going to have to pay the amount. That whole penalty doctrine thing is not applicable here.

What the posters above are basically saying is that the company will not waste the time to try to enforce this clause against you. On this, I don't know how large this Firm A is, but if they have their own inhouse department, it will cost them nothing to send a Letter of Demand to you. It is also very easy for them to make a claim in the small claims tribunal against you.

Best to try to settle it amicably, otherwise please know what you are getting into if you ignore their requests for payment. Things might go well, they might not. But legally, you are definitely in the wrong.