What they charged you was not a fee but a penalty charge, because you broke the terms and conditions. This point seems to be important because if they can prove it was a "fee" (i.e. a charge for a service) then it is harder to claim back (i think!).
If it was actually a penalty charge (which it was) then these are unlawful at such a high level and so can be claimed back much more easily.
I think we are in the bizzare position of having to prove we have broken their T&Cs!!
Someone please correct me if I am wrong.