I don't know if anyone else out there has tried this site, but it has been really helpful for me. Being a 3.2 owner and living in the middle of nowhere its helped me track down super unleaded and also find the cheapest petrol.
Give it a try, you will be amazed how much you could save over the course of a year! (Especially with the way petrol prices are going!!)
Must add I am not connected to this site, but highly recommend using it. It's free too...
You get 20 searched a week, although you can search as many times as you want using the same post code etc.
Let me know what you think.
(awaiting lots of tight scot etc jokes........)