Hi Paul welcome to GRG,

With regards to your problem where do I start? A few things to consider are what mileage are on your trainers, have you measured your pronation, do you warm up/cool down after each run, do you do any stretches. Could be any of these and more that others will no doubt ask you, from that we may be able to indicate some probable causes and suggest a solution.

Derek

I agree with Helen that it could be your ITB.  I had a problem with this after doing some long runs and it was quite severe.  However, after doing a week or 2 of the correct stretching it was completely cured so this could be good news!