I'm not sure how you've calculated the interest rate, so its difficult to say whether its right or wrong! It is a fairly high amount, but if you are claiming that amount of charges back from prior to 2003 then it is quite possible that it is correct.
For example, if you were charged £1753 on January 1st 2003, the combined interest to date at 8% PA would be £530.94. If you're talking about charges going back before 2003, it is quite feasable that the interest you have calculated is correct. Have you looked into
contractual interest?
If you want to get your charges back and a
whole lot more, this may be for you! Using the arguement of "mutuality" or "reciprocity", it is quite possible for you to be charging your bank the unauthorized borrowing rate, instead of the 8% that is allowed under s69. CCA. If you did that, you could well find yourself claiming back in excess of £2000 in interest!
If the banks can do it to you... Why not do it back to them!