Bear in mind the timescales as your mortgage ended more than 6 years ago.
"If your policy ended over six years ago: The ‘statue of limitations’ means banks don’t need to keep records that are over six years old. However, there is no official cut-off time so if you’ve still got the paperwork, while your chances of success are a little lower with older loans, many still dosuccessfully reclaim."
Most PPI was on credit cards. If you had it on a mortgage it may be harder to show it was mis-selling
This site is useful
http://www.moneysavingexpert.com/reclaim/ppi-loan-insurance