Dear Sirs,
If you wish to pursue this claim (which is disputed), then you must issue Court proceedings.
Any further correspondence from you, concerning this matter, will be ignored.
Yours, etc.
1. Unless your original agreement makes you liable for such costs, you have no liability whatsoever.
2. If they do issue a claim, it will cost them a lot more than the amount in dispute. After wasting their time, and causing them to run up costs, you can just pay up (and they have no right to claim costs from you, because the action is a "Small Claim".