The running shoe is a small part of the sum though...most of the running retailers are exellent. - Runners Need & the London Marathon Store in particular. Most of them guys are hardcore runners and marathon runners, and some Ultra-runners at the LMS. I would say, having worked in the biz a few yrs back, that 95% of the time, injuries are due to the runner and not the shoe...so don't expect the shoe to be a miracle cure BUT I hope it helps!!
It would be better going and getting a physio to look at your running style and point out any weaknesses - most people don't stretch or do strength work. get well soon.
MULTISPORT - Does the half start at the same time and place??? Is is also simular to hastings - which i normally do before London...and in you opinion, would you do the 21 a month before London or do what you normally do (which is a hastings style half)