Data charges

I cancelled my contract on a certain date and I have also received an email to confirm my contract will be cancelled effectively on this day. However I have now received a bill for after this date and I have also notified autopage my line is then still running. I then queried this to state I am not paying for out of bundle rate for my data as I was refused to be loaded a bundle as my line was already cancelled. I have everything on mail and Autopage is phoning me consistently for the invoice to be paid. but no one is investigating what happend