There is Up & Running in Watford. I receantly bought shoes from there and was pleased with the service, and they 'assessed' my running. also look for a shop that lets you go for a run in the shoes, this is always useful so as to tell if they really are a good fit.
PM thanks for the info, just wondering if you have a large range of abilities, what with just starting out and all! currently running about 30 mins at a time, hoping to extend that over the next few weeks.