Overcharged Interest on Balance Transfer

I opened a credit card with Texaco - to do some tarting in May 2005 - I transferred over 3 balances, but only 2 were put on the deal I wanted - 3.9% for life of balance. Stupid me - didn't realise. Account was transferred over to Lloyds TSB in July 2006 with no change in terms. I started receiving paper bills 12 months ago, and then realised I had been paying too much interest on 1 of the amounts I had transferred over. I have been battling with Lloyds TSB for 7 months, trying to get them to sort out the incorrect transaction that had accured in June 2005, but they keep fobbing me off saying someone is looking into it and they will notify me when they have a answer. I phone them every 6 weeks on an 0870 no costing me a fortune, only to have to explain the story again and again every time, sometimes to 2 or 3 different people on the phone. I need help with this, but I don't know where to start. Can anyone suggest anyting positive.PLEASE