Try Office of Fair Trading website for info -
www.oft.gov.uk or call Consumer Direct to speak to someone - 08454 04 05 06 - they can help. There are strict rules about contracts, unfair terms and changing contracts. Try OFCOM also, they regulate mobile comanies.
THEY have to prove the advised you of your changes, ask them for a copy of the letter they supposedly sent you or you will request ALL your info from them under the Data Protection Act. An advice sheet won't cut it! They have to provide you with your new contract so you know what you have entered into. Trust me on this - follow the 10 min rule....if you get no joy from who you are dealing move straight to a manager and don't give them any leeway. Hope this helps, best of luck!