Breach of contract

I recently came to the end of my contract and did not intend renewing because I was moving house in the same month (August). I went on to VM website to notify them of my intent, on logging in I noticed there was a new offer for me to stay with them. I looked at the offer of £27 per month and I accepted because it was a good deal and cheaper than I was paying, they got me up and running in my new property and all was good. Until I got my bill that is, they are charging me £65 per month, I contacted VM and they said that that offer is no longer available and went on to say best they can do is £36. I have the contract they sent and it clearly states £27 per month for 18 months, how the he'll can they justify a 60% increase? How are people supposed to manage finances and budget when VM pull stunts like this?. Am I able to pull out of the contract? I just want to go with BT now and be done with them.

Joe