Good man, but be prepared for a fight, even a CCJ.
I agree!
Opti be very careful with cancelling the DD and so withholding payments that, at the moment, they are LEGALLY entitled to.
The best course of action is to write letters laying down exactly why you are cancelling your contract with them. List everything applicable, then express your conclusion that they have not honoured their contract with you by failing to provide the service that you should reasonably expect them to do. Claim any damages, such as costs in attempting to get them provide the service they did not, and miscellaneous charges such as cost of letters and phone calls. Throw the legal challenge to them and give a reasonable period of time for them to resolve the matter, such as 14 days, before you take legal action against them for a failure to provide a service as per contract.
Also seek advice from Citizens Advise, or a solicitor, who's costs you could add to your final claim.
That is my advice, but do please ensure you honour their contractual charges first before taking your action against them