I have had the same problem. They offered a really good initial deal of approx £200, then called me up at renewal time. I had checked the contract and it had stated that they would auto roll on the contract if I didn't give give 30 days notice 90 days before the end of the contract. The contract also states that they would only increase the fee by 10% each year. I thought that I would continue the service.......until the call "the cost is £3500 you have to pay now!!!!"
I spoke to trading standards, they said that they couldn't help a business and that they only helped consumers. But after a long conversation with them they advised that the increase in costs being excessive (over 3000%) for no additional service that a court would not likely uphold their claim.
The upshot was don't pay them anything, don't give any permission for them to do so, then let them chase you for the money as it would be a default of a business invoice, let them take you to court which they are are not going to do.
Important:
You can legally request by letter or email that you do not want to be contacted by them by telephone and that you only want to be contacted by letter or email.( I would send a registered letter)
Also you can legally ask for all copies of recordings of telephone conversations made between you and them. They must supply them within 10 days. This usually slows the dogs down