Hi, ring up the number on your letter (the one that states PIP Mobility).
Ask them for a "Certificate of Entitlement" letter. Will take about two days to arrive. THIS IS WHAT YOU NEED.
Then pop into a post office with your log book - they will check it over and send it off. It will take about four weeks. You'll get your log book back stating - disabled - then a few days later you'll get a rebate. £180 paid in September 2014 - 9 months has passed - £70'ish - you may get about £30'ish.
The rebate will only start from the date you send in the information.