Don't want to sound daft, but in France, there are 2 main websites which indicate the prices of fuel (all types) for all stations. The first belongs to the government, and the second is more interesting. It is privately owned but can be updated by any of its members. It's a community so to speak
. One can register for free, enter his location and the website gives in return the best prices around. At any time, one can enter the prices observed around. The website gives the date at which the information was entered. The website also can give the best prices for an itinerary that would be entered. It also shows the location of the station on a Google based map. The member can also set a list of his "favourite" stations with a direct link from the home page. Finally it has a mobile phone optimized page. Have a look here (it's in French and for France at the moment):
http://www.zagaz.comAnything like that in the UK? maybe a good idea to start something like that in the UK if not available...