Thanks to all for great advice recently re shoes. As promised I am posting to let you know how I got on.......
Had a great three days in London with my wife, saw Angela Lansbury in Blithe Spirit, met new nephew aged 2weeks, saw parents in law etc, ate far too much and spent even more!
I choose sweatshop as per lots of your posts and the guy at Rathbone place (just off Oxford street) was very helpful and was happy to help an overweight novice. Really put me at my ease and I came away with some great Brookes with moulded in-soles. (I was over pronating etc - watched the before and after videos etc)
Home now and ran in them for first time today and knocked 20 seconds of my best 5km time!
Thanks again for the advice.