I would agree with pakkasham, quite often these issues come from higher up. When my pelvis gets bad enough my feet set off eventually!
Personally as well as physio (which I need for other bits anyway) getting the ligaments working better really helped- I have mostly barefoot style shoes now.
The trouble with riding is that it does put the aggrevated area into constant stretch from having our heels down. I would also freeze a bottle of water and use that for rolling under the bottom of your foot after riding if you can/and evenings.
