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+1 Runners Need, Canary Wharf. If the same guy's in charge he's a keen runner and distinctly non-salesman-like, in a good way. They also have a good price-match policy against online goods.
I think the jury's still out on the value of "gait analysis" by treadmill/video, but short of having a proper biomechanical assessment with an expert, at least it gives you a chance to try on a number of shoes from different manufacturers whilst actually running. Go for the ones that fit best and feel most comfortable.
Avoid Sweatshop, and their sales staff who are incentivised (I believe) to try and sell you Karrimor shoes and/or expensive insoles.
I found the Runners Need in Canary Wharf great. Jumped on their treadmill, recommend shoes to correct my Supination. Would bang again.