they are very sneaky at adding interest where we think there shouldnt be, but its all in the small print.
I ran up a very big CC bill once and when i finally had the funds to totally wipe it i called and asked for the final figure and transfered the money that day over the phone to clear it.
a month later i got a bill for over £100 interest, when i called them they said that although they have given me a figure to pay it was only what system was saying was due and not including the interest from last statement date. I stood my ground, i had asked for a final payment figure, not a statement figure, and eventually after a few calls and frustrating conversations i got the final £100+ wiped.