Turns out I accidentally only paid half my credit card bill last month. Until recently I had a Bank of Scotland Mastercard which I used because it supported a good cause. Then without any input from me it transitioned to an MBNA Mastercard, new number, which doesn't support anything except MBNA. MBNA prints its statements on an A4 sheet of paper, but the column of figures stops halfway down with a subtotal and is carried over onto page 2. I had just paid the subtotal balance of the figures on page 1. Quite comfortably over the minimum amount, but twelve pounds interest? Bloody hell, so this is where credit card debt comes from. I can safely say the full sum has now been paid in full.
So, they don't support my charity. They cunningly rephrase the words "available credit" on the statement as "available to spend", which doesn't send exactly the same message. And an employee of theirs was an appallingly bad husband to the daughter of friends of my parents. I may change cards before long.
(That last reason may not technically be a fault of MBNA, but if you (usually) pay the balance in full then there's no material difference between any of the cards on offer, so you have to base your decision on something.)
Anyone who points out that if I kept better track of my spending then I would have realised the page 1 sum was too low may now leave.