In reply to Dan Arkle:
It might not be PF. Professional diagnosis recommended. I had an outbreak 5 weeks before setting off on the John Muir Trail so I had to fast track diagnosis and treatment.
One visit to a physio who helped with a stretching regime. Visit to podiatrist who made custom insoles to correct gait.
I had about two weeks before departure to get used to the insoles - you need to build up usage.
On the JMT I took ibuprofen for the first 5 days but then never required more and I have had no further problems.
I now only use the insoles when doing proper walking - not for everyday use. I didn't get on with the strassburg sock (couldn't sleep and it hurt my ingrowing big toe nail), also my podiatrist wasn't keen on them.
Headline conclusion - custom insoles from a good podiatrist