Clare, Thanks for this post.....I'm going to try it and let you know how I get on!
I'm really glad to hear it has sorted things for you - must be a turning point for your running. 
I've been exactly the same as you - it's plagued...(dare I say ruined) many races and training sessions for the past 10 years. Hadn't thought of trying Fybogel, but perhaps it works by regulating the bowels properly.
You cant imagine how glad I will be to run even 3 miles without having to go, so I'm willing to try anything which may work!
Am hoping they sell it in my local pharmacy.