Andy, I've had good service from both Novatech (
www.novatech.com ) and PC World (
www.pcworld.co.uk or
www.pcwb.com ). I suppose, if you're going to use PC World, you might as well go into a store, but their online prices were pretty competitive, and their customer service has been excellent every time I've used their site.
Someone might come along with some cheaper suggestions, but these two are normally there or thereabouts when you check on Kelkoo or similar.
Cheers,
Ian