Question about punitive charges

nikmacve

This question is not strictly about bank charges but I haven't found a more suitable forum to put it in. BT are trying to charge me £5 for late payment of my bill. It was paid late because I disputed a charge of £115 they applied for their engineer to visit my house and fix THEIR equipment - they refunded it but it took ages to actually get them to do so. As any extra charge can only cover costs, and the delay was because of BT gave me an incorrect bill, are they allowed to charge me for a late payment?? If I calculated my costs trying to resolve this they would surely be more than a fiver...