Ian, Cancel your direct debit immediately, do not pay the bill, let them argue it out, tell them you are going to deal with ofcom and then report it to ofcom, unless told in writing that vodafone have an arrangement with cable and wireless you should be entitled to assume that the roaming agreement is with manx telecom and not any pirate that feels like ripping you off. Get another contract immediately with a different network so that you still have a mobile phone. As your network provider they are oblideged to use the cheapest or most cost effective supplier for you when roaming.
Russ