help with o2..

Basically, i'm a fanny. I cancelled my pre-pay contract with them a couple of months ago then realized that they were still charging me (charge was to a different account). They sent me a pay as you go sim but i forgot to put it in my phone as i got a new one from a different company so they're saying i technically still had my contract running even though i wasn't using any of the minutes or texts. I've put in my sim now but they're saying they're not going to refund my £120 (argh). Anyone work for o2 and know anyway around this or just got any ideas in general?

And yes i know £40 a month was pretty steep but i used shit loads of texts.