@ intexatthemoment: A helmet needs to provide a good fit. Otherwise, at best, it could prove a waste of money; at worst, it might cause more harm than good. As per mynamehere, any good cycle shop will talk you through helmets and, more importantly, ensure you obtain the snuggest fit possible. We have our local BC Bikes on East Dulwich Road (which comes recommended on here - though I have never used them). Personally, I can recommend Brixton Cycles for advice/service/range etc. However, the downside with lovely specialist bike shops is that they can sometimes be pricey. Decathlon is less expensive, has a massive range and gives advice too. It is, however, all the way in Canada Water. Having said that, I think the trip is worth the effort.