Hi,
I don't know if they have, but if it was a balance in arrears, you would normally have to pay interest (like a loan, you pay interest until the day it's cleared in full).
By the by, I cancelled my MBNA card too, in December, paid the government tax on it for 2007/08, and got 2 letters confirming account cancelled (I requested them as having things in writing make it much easier to sort out problems afterwards..as occurred in fact). Then yesterday I got another bill, with more government tax for 2008/09. (which itself is even wrong, as that tax is paid in arrears, not in advance).Grr!!
Maybe phone them back to double check if you're still not happy?
Nicola