Even though the network is three,your contract is with the 3rd party, it definatly seems as though they have breeched the contract, therefore you are entitled to cancel the contract, make sure that any documentation you send is sent recorded delievery.
If you do take it to small claims court, there is a strong possibility that you will get the costs back.
Basically send a letter cancelling the contract ( if thats what you want to do), and also state that you have follwed all the correct procedures regarding the refund, as they have breached the contract you have no option to to proceed with court action, for the refund oweing, and any costs incurred, but as a goodwill gesture you will hold off court proceedings for 14 days.
Good luck