Mortgage Express buy to let added interest at end of mortgage

Nippychick

I am coming to the end of the term of my buy to let mortgage. Having asked for a redemption statement, a further £372 has been added which mortgage express says is for a payment in june 2005 when they decided to change the way the mortgage interest is collected to comply with industry regulation. This has never shown on the closing balance when I received my statement each year. I have queried this and asked if they told me about it at the time. They have replied that at the time they took a business decision not to inform customers but as they didn't think it fair to charge 2 amounts in one month they set the amount aside until the end of the mortgage term. This has come as a surprise. Can they do this? I know I can complain to them but if this is not resolved is there another body I can complain to. Thanks