In reply to JimR:
If your pain is mid arch and big toe, then it may not be PF.
I suffered from a very nasty bout of PF as a running injury which got worse and worse over several months, to the extent that I couldn't walk. Physio didn't help at all all. However my physiotherapist suggested cortisone, and wrote to my GP.
The fascia ligament generally tears at the heel connection causing underfoot pain. It's a thick band of fibrous tissue not well served with blood vessels and natural cortisol, hence the value of an injection. I imagine you know this already.
You may need to get a proper diagnosis but in the meantime you might find a Strasbourg Sock will be invaluable. It keeps the fascia in the elongated position overnight while it rests and has a chance to heal. It could be twenty quid well spent