ADT's FAILURE TO CANCEL CONTRACT

I moved out of my house on 27 February 2014. I telephoned ADT to cancel the contract and was advised to fill in a form. I filled out the form and sent it to them. I didn't hear anything from them & assumed all was in order. I needed my bank statement's for an account so I obtained same from the bank and then realized that I was still being debited for March, April, May and June and placed a stop on July's debit order. I contacted ADT on 2/7/14 and sent further correspondence.I then received a call this year from debt collectors saying that I had been handed over for 1 month's instalment.ADT says that because I cant send them proof of the initial mail(because I left my previous employment last year March I no longer have that mail) I must still pay the instalment that was due on 15/7/14(e mail detailing that ADT was still debiting me despite my cancellation request was sent on 2/7/14).apparently for 15/7/14's instalment to be cancelled I had to have proof sent on 6/6/14. I am totally frustrated about this.