Urgent Help, Please!

1 post

I took out an 18month contract with three in July 2007. I took the 500 mins and 100 texts for £20 per month, with a Nokia E65. Over 6 months, I had extremely poor signal, where it consistantly dropped signal in areas which other 3 customers had signal. In January 2008, I decided to cancel the contract, where I was told that the remaining balance was £240 (12 x £20).
I have just had a letter now stating that I now owe them £359.34 because I decided to cancel the contract meaning my price plan goes up to £30 per month. Can they do this, as I signed a contract stating £20 per month.

Thanks