Service OK until contract runs out & you wish to leave....

I would urge anybody NOT to sign up with OneBill Telecom as I am going through hell with them!! I had a 2 year telephone contract with them and decided when it was up I wanted to switch providers - I gave them plenty of notice as in their terms and conditions AND I kept to my full contract length. HOWEVER, they then send you a cancellation invoice for £300!! Firstly, there shouldn't be a cancellation charge if everything is done correct and secondly, this figure is extortionate!! I have spoken to one person on the phone at OneBill and they are simply arrogant and bullying and provide no help whatsoever! So, I have tried to contact them with regards to this by email several times and I haven't had a single reply....just more automated bills, and another one today for another months payment plan PLUS the £300 - i'm not even using their services anymore, so how can they charge me for it?!?!?!?!?! I need to wait until November 30th for the Ombudsman to be able to look into my case and I have also contacted Watchdog - I can't believe this company is still allowed to operate.