This site is maintained by Fubra who are the people behind OurProperty.co.uk. To find the lowest petrol price in your area you just type in a town or postcode. By default it looks for unleaded prices within 5 miles of your area. You can change the fuel type to Super Unleaded, LPG, LRP, or Premium Diesel and the distance to 2 miles (inner metropolitan areas only), 10, 15 or 20 miles. As well as listing the 5 cheapest stations and the price, you can see their location on a map from Google Local.
The data covers over 10,000 stations and there are about 8000 daily updates. Data is provided by Catalist in asssociation with Arval and collected from fuel card transactions that are processed at petrol stations across the country each day.
You can search 20 unique areas or postcodes per week and are allowed to search for each unique postcode as many times as you like. It is a free service but you do have to register to view the detailed information.