Reclaiming bank charges: Is the Nationwide building society a 'bank' that might have to repay illegal charges?

I am currently in the process of drawing up a letter which - I hope - will allow me to reclaim the monies owed me by Nationwide Building Society (excessive charges levied since 2003). However, would I fall at the first hurdle because they are not, essentially, a bank? If you're someone who's already been through such a process (importantly, with the Nationwide) then I'd love to hear of your own experiences/outcome. Thanks.