Hi,
I had the same problem, amongst a few others with 3, I checked the on-line info and they have amended there terms to prevent people down grading.
I wish you luck as I spent many hours on the phone to India fighting a variety of poor service issues and 2 written complaints to their head office, neither of which which were answered by head office and without any favourable resolution, this despite me being on there highest tariff and being a customer for several years.
I have attempted to hand my notice in twice, to be sold to in the crudest manor, telling me that I'm one of there most valued customers, whilst offering me a cheaper tariff, "special one off deal for you sir", which actually turned out to be a package which is being sold on-line, it just has fewer minutes....
the question I ask, is it legal to change a customers terms and conditions without notifying them? pretty dodgy if you ask me...
I hope you get somewhere with this.