nat west int only mort

Smokeyone

I would appreciate a spot of advice on my nat west fixed rate int only mortgage. I received my first statement today and it seems I owe them more than when I first started. After much paperwork searching I think I know the answer. They quoted and deducted each month the interest on the amount borrowered but they have admin fees etc for which they have charged me interest over the last year but did not include in their monthly quote.
It's not the exact figures but as an example £100K at six per cent equals £6k divided by 12 equals £500 per month. This is the amount paid each month but nat west also have admin fees so at the end of the year I now owe them a chunk more than when I started.
It's my own fault but do I have any legs to stand on..............................