Hi - Well done getting those charges back, they really shouldn't overcharge anyone in the first place and they know it full well.
If they applied this way of thinking in the beginning, then they would not cause all this resentment from their customers.
These actions only lead to more paperwork and hassle for themselves, and it has a knock on effect leading to bad customer relations and invariably customers leaving them.
In fairness though try not to give them reasons to overcharge you again at all, prevention is better than cure.
First direct may close your account if they so wish, but I don't think they want people to close their accounts at the moment, due to their announcement to charge customers for current accounts. But if they do it's not the end of the world there are many other alternatives. So don't really let that alone influence your decision.