I am an in a similar position. I am owed 600+ for charges from 1997 to 2001. I successfully claimed a similar amount for charges 2001 to date, and have started a separate claim. I am at the stage where I am about to issue proceedings, having received an offer for £300.
Although the Limitations Act would appear to prevent you from claiming charges over 6 years old, there are exceptions written into the Act that cover deliberate concealment, and errors. In other words they must have known that the charges were unlawful at the time they started to apply them to our accounts. I cannot believe that their legal people would not have been involved at an early stage, and they would have known all about the case law and consumer law that existed at the time, so it is a reasonable assumption to make that they have in fact concealed this information, and therefore the Limitations Act does not apply.
However, making that assumption and actually proving the facts are 2 different things, especially where courts are concerned.
But on balance, I am prepared to issue proceedings, even though the ground is a little thinner, because I still do not believe that they will defend their position in court, and if by some miracle they do put in an appearance and successfully defend, all I will lose is the £80 court fee