You'd be much better off trying to spend a few extra quid and get something recommended by a proper sports shop rather than trying to save yourself some money and getting a reduced trainer that doesn't suit your running style from somewhere like JJB or Sports World.
Just go into the shop and be honest with them, tell them you're on a limited budget and i'm sure they'll do their best to help you! My guess would be that you're looking at £40 upwards for a trainer recommended by a specialist store. Hope that helps!
