Need help trying to cancel free offer from SKY nightmare

I wondered if anyone can offer any advice, as I'm stressed out and worried now we have agreed to something we can't get out of.

We left Sky last year after 15 years of service, a pushy rude sales guy in the end did it for me, Virgin have been great ever since and really happy with the move, although I think the SKY TV interface is smoother, Virgin's super fast BB and Tivo box are wonderful, plus we got a great deal.

However Sky rang last month (as they have been doing ever since we left) and this guy offered me a too good to be true deal, normally I say no, but he said £20 a month for everything inc Sky Sports, plus free for 2 months and on a month by month contract so I can cancel whenever I like. So I though as I've been buying Sky Sport pass to watch the F1 I'll use the 2 months and we can check out the new Encore channel.
3 weeks later we have decided to ditch Sky, we never used it, it was nice for the Sky F1 but not worth £20 plus it doesn't include HD, so worthless for me.
But I had a 2 hours!!! live chat with a a Sky rep who says we can't cancel without paying for something that was offered for free, if I cancel now she will remove the credit and we have to pay for whats been used! I explained but we never used it and the deal I agreed to was on a month by month basis. In the end I had to leave it as she refused to do anything else, I asked for a manager to phone me back, but one never rang:(
I can't find any contact email and refuse to pay to use their premium phone line as it would cost a fortune if on hold that long. I'm waiting again today for their online chat system to be available (takes forever) hoping I can get someone to ring me.

Has anyone got any other advice on how I can solve this? or contact Sky?

I've looked for their help forums & I've emailed direct to a CEO email I found, but had no reply. I'm really concerned I've been conned into a contract we didn't want or need.