Unfair late surcharge after account cancellation

[SWFranceMax]

Hi - I cancelled our O2 account after they advised us that continued use would be subject to added roaming charges now that we live permanently in France. They advised that the account would close after 30 days. Literally the day before the account closure, they notified us that they are applying a surcharge for unfair usage, I assume during this period although we have switched to a french mobile account. Never mind 'unfair usage', in my book this is 'unfair application' of this rule knowing that we were leaving. The sum was not high per se (£10.24) but that is two thirds of the former plan cost and I'm sorry, but it's just the principle! Grateful for any advice. 

[MI5]

@SWFranceMax 

If you are still connected to O2, call on the free number http://www.o2.co.uk/contactus to see if you can get refunded.

If your account has now been cancelled, just pay the bill as it will be cheaper than trying to call from France.