I heard that it was owned / run by one of the companies that owns a lot of the firms quoted on it. It doesnt include smaller local firms (tho its hard to work out which are which given the numerous 'local' trade names they operate under.
They were criticised on Watchdog I think for not making this clear - I dont know whether they have improved the site to do so....
It is a useful tool I think but I would make a couple of phonecalls to local suppliers as well (we found a lot of variation and saved about 7p a litre :-O by making a few calls,.