Having problems with a company called Sat Cover ltd taking money from my account without autherisation?

I took out a extended warranty for my sky Equipment with a company called sat cover ltd. At time when took out cover they told me they were part of Sky. But I have found out recently they are not and have been taken to court by sky. the last few months I have been paying �6.50 a month. when I found out they are not part of sky I phoned up and complained and asked to cancel. They won't cancel and now have changed my direct debit with out my permition or authorisation to �65 per month and have taken �65 from my account this month. im really angry . i phoned them to try and complain but can't get through they just keep putting you on hold and then hang up. I have phoned up natwest who were not that helpful and said there wasn't much they could do and that I need to contact company which obviously I have already tried to do. The person I spoke to at natwest gave me the contact detail of this company that they have on there records but would you believe it they only have an address no telephone number. The address they gave me was the same as I already had. Natwest said all they could do was cancel direct debit. What can I do to get the money back that Satcov ltd has taken. isn't it covered under the direct debit guarantee. Don't my bank have to give my money back under these cirstances or is it ok for companies to help them selves to any amount of money they want without asking. Also I want to cancel the direct debit for this company but am concerned that if I cancel it now I won't be able to get my money back. I don't want them taking any more money out. Should I cancel direct debit, will I be able to get money back if I cancel direct debit or is it best to wait a bit. Any advice would be much appreciated