Thanks for your advise. Are these charges "unlawful"? How do I prove this?

I have recently sent off my initial letter to Barclaycard claiming that I had been missold my PPI on my Credit Card. they have responded with a letter containing a questionnaire. Some of the questions refer to how I made the original application etc... Sadly I do not remember a lot of the information they are requesting. Is it my responsibility to answer these questions (even though they presumably know the answers themselves) or should I ignore it and go to the next stage? In addition I would like some advise regarding over limit charges. I have ALWAYS paid the minimum repayment on my card each month. however, at some stage a year or so ago my card has gone over its limit. This must have been to do with charges as I have not paid for anything on the card for many years. As my direct debit is instructed to take the minimum payment surely this should always take enough to avoid my account rising above the agreed limit. I am now being charged £12.00 per month along with PPI insurance and interest. It is staggering and because my card is over the limit I am unable to take out a 0% deal with MBNA. My credit file is otherwise unblemished and (for what its worth) currently reads 999/1000 with Experian. Please advise!!!