Many runners end up needing to see a physiotherapist at some stage in their running careers. Ideally, you should see a physiotherapist with an interest in sports injuries, and preferably particularly in running. Some physiotherapists will treat the symptoms of a running injury, without seeking to address its underlying causes, and it is important that any advice you get is related to the fact that you are a runner. Depending on your injury, you may want to see a podiatrist.
Podiatrists diagnose and treat disorders, diseases, and injuries of the foot and lower leg.
The Serpentine Running Club has a partnership with Medifix , in which all new members receive a free pain and injury screening, and there is a free fortnightly drop-in clinic at the Wednesday night runs. See here for more details.