Brief summary/history Other half's card. Commenced 1998, defaulted 2006 with interest frozen from that date. Default card balance was £6,500 - progressively paid/reduced to just under £5,000. PPI claim made October 2012 - initially refused but upheld by the FOS on adjudication. March 2014 NatWest credited £4,664 to the card, leaving a balance of £323. By my own (spreadsheet) calculations the payment should have been some £1450 more than they paid. Their payment was arrived at as follows: Refund of payment made 1998 to 2006 £2334.14 Compound interest based on rates charged £ 712.36 Sub total £3056.50 Gross interest at 8% £2010.43 Less income tax at 20% £ 402.09 Net interest £1608.34 Net offer £4664.84 It is their compound interest figure that I cannot agree with. During the 8 year life of the card, monthly interest rates varied between 1.385% and 1.620%. The average card balance was about £3200. My calculations show the compound interest figure to be about £2077 rather than their figure of £712. This would also increase the 8% gross interest amount due by about £1000. I reckon we want about another £2500 from them (before tax) I have written to them several times asking for a breakdown of their compound interest figure and have been stonewalled each time. They say they cannot provide these (complicated) calculations but that they are in line with the FOS and FCA guidelines. Their last reply even went so far as to 'explain' to me how compound interest works. I can scarcely believe the content of it. Here is the relevant paragraph from their letter: As a basic look at the compound interest aspect, if you compared the monthly balance of the credit card with and without PPI and the difference was £20, then the interest element would be determined from this figure. If the monthly interest rate was 1% and 12% per annum, this would mean that the starting figure for the compound interest is 20p (1% of £20.00) if your card then ran for 5 years, the compound interest on this PPI premium would accumulate as follows, year 1 = 22p, year 2 = 25p, year 3 = 28p, year 4 = 31p, year 5 = 35p. Therefore the total amount difference in balance of £20.00 in this example would be 35p. So, this 'expert' from RBS thinks the compound interest would be 35p. Well, I calculate it to be £16.33. She seems to have worked it out at 1% per annum rather than 1% per month. (I am not a mathematician so if I have got this completely wrong and she is right, please, someone, tell me I am making a numpty of myself). So, ladies and gentlemen of CAG, where do I go next with this shower? Suggestions and comments most welcome, thanks.