Yes I had the same problem. I borrowed to pay off all my balance in October 2010 to take advantage of their "0% deal" and was surprised to see massive interest being charged. They actually had the gall to say that it was because of a previous late payment that took place before the promotion and before I had fully paid off the debt!!
I now have the problem that one inadvertent late payment is going to cost me hundreds of pounds. I have written to them to reconsider and with an acceptable payment plan but I suspect that they want their "pound of flesh" because they have already called and e-mailed for "arrears" on interest that is not even due yet
On principle, I am perfectly ok to pay off my 0% balance (despite the "transfer fee") but when you are met with this inhumane "we can't do anything - pay up now" attitude, they really make you want to fight back on the same "unreasonable" terms. I'm glad I found this site to fight fire with fire!