This is a site that I have used for some time and it does save me a little money. It is a service that collates forecourt petrol prices and shows you the cheapest near you or where you are visiting.
The data is acquired every time someone uses a fuel card and the popular stations are updated daily. The price tracking is not quite in real time but if you subscribe to the custom daily email price update and you will see which of the stations is normally cheaper than the rest.
You can not check the price of a specific station and you are limited to checking the bet price in 20 places a month, but you can recheck each place many times so that is probably enough for you. Check out the free service at http://www.petrolprices.com/.
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Having read this I wondered if you are interested in a map I have showing petrol prices in any area of the UK. It relies on users to submit the prices.
Take a look at http://www.whatgas.com and let me know what you think, we are now letting people feature this map on their own website if they like!