Barclays Bank

I used a balance transfer offer. The agreement was that they would not charge interest, and then they started charging interest, claiming that it was on my other purchases. I reduced the balance substantially below the original balance transfer amount and the still kept charging interest. Further, they have raised the interest charges to about 20% even though I was never late with a payment. I just paid off the bill. I plan to cancel the card. Shall I try to resolve it first? It will take me a week or two to pull all my old bills together.