Vodafone overcharging on contract help - cancellation/downgrade?

[Minime]

Hi, I joined vodafone last august on a contract that said i would pay 25.54 a month for 900 mins unlimited texts and unlimited internet. this looked like a great deal so i took it with the samsung tocco ultra phone free. not once since i took the contract out however has the price been under 35 pounds. it has reached 56 when i went abroad (but hardly used the phone) and averages at aobut 42 for a normal month with only a few extra charges (should add to about 3 pounds.) i have no idea where the charges are coming from. what are my rights regarding this? I would like to change to a Blackberry phone if possible, but my contract still has another 14 months left! firstly, is there a way i can cancel this contract or downgrade it dramatically? also, would it be possible to use my vodafone sim in a blackberry if i just bought the phone sim free separately?has anyone else had similar issues