Thank you for your reply. I'm pleased to say I haven't really experienced problems for a while now. As it happens, I was helped by a physio when selecting my last pair of trainers. I bought some New Balance ones in the end (we have a NB factory outlet near us which is rather handy).

2 weeks ago I did the Edinburgh Marathon and I had no problems with PF either in the race or during the 18-week training program I followed. My only problem was very tired legs!!