Claire, veto what the others said about running on the old trainers for the race. With regards to new ones, Sweatshop shop are not bad, and they do have a footscan analysis system to tailor find your perfect running shoe. There's one on the high road in north finchley, about 200yards from the tally ho pub. The best shop i go to is runners need in camden, and they have a new shop in liverpool street. They use video footage analysis with you running on a treadmill, just as effective, but the staff in my opinion are more technical and knowledgable, and they have more shoes and variety tailored for running. If you are a member of a running club and show your membership card, there is also a 10% discount at the sweatshop too.