NO assistance when it is not our fault

My Dad wrote ADT a letter in September 2013 to say that he will be going on pension in November 2013 and wants to cancel the contract he has with ADT 1ADT07242. A consultant called him back to say he must not cancel the contract, but he will receive 6 months free cover as he has been a loyal customer. In February 2014 my Dad received a call to state that this information was not true. He has subsequently written letters and emails to ADT explaining the situation. From 2015 he has received sms's and calls from both ADT and the Nimblegroup asking for hte outstanding monry. Every time we send emails to bioth parties explaining the situation. He has now received a letter of femand form the Nimblegroup stating that they will take him to court for the outstanding fees that have now been tallying up due to the interest. Everytime he has spoken to a consultant they say that they will close the account. Out of principle I do not believe we should pay this as he paid upto the end of when his cancellation was due to take effect, in actual fact more than that.