I've used Megazorb for years in litter trays and have never had a problem with a 'smell', although some people do say it smells. In my opinion, it is THE most absorbant substrate and I've tried loads! I would never use it without a top covering of hay though, otherwise it gets spread everywhere.
I'm afraid I don't live in Twickenham so I don't know of anywhere nearby that sells it but if you go onto the Megazorb website, they have a list of stockists. You may find a local horsefeed merchants will be able to get it for you if they don't already stock it.