IMHO the first thing to do is ask for a copy of the contract with your signature on it. If there is no signed contract then tough on them, if there is then see what the termination terms are and how long the contract was for.
Have a chat with citizens advice or if you have free legal help included with your bank account, professional membership, house insurance or similar then you could ask them for help, they sometimes assist you with writing a snotagram with suitable wording that means "up yours".
A sneaky way find this out is to phone AMCO and hint you are thinking of changing your contract from ADT to AMCO (who are half the price) but state that you need to know how to cancel your contract with ADT, with any luck they might tell you.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates