10miles is bigger than Edinburgh is wide, as the crow flies. No way would I drive out of Edinburgh to save 3p!
As people have said it's not price fixing. Drive out of any major route out of a city and you'd notice that they have put up the pricing. Why? Because people will still pay for it. If they really need the fuel then they have no choice. It's like motorway petrol stations. I can think of several petrol stations across Edinburgh that does this, even a couple of the "city centre" charge a premium centre rate. Just like how a RSMcCall (or even a locally own corner shop) charges more for a Mars Bar city centre or at the train station than they do else where.
Every area has their own micro-economy that adjusts local prices. On a much larger scale it's like the Euro, each country's Economy is different. Go to Spain and write to the PM why a pint is a different price than it is over the border in Portugal. You wouldn't, yet it's being paid in the same currency.
There's also the cost involved of transporting the petrol, again which causes differences. Penucuik 9.8miles outside of Edinburgh is more expensive, Peebles at 23miles is even more "remote" and even more expensive.I would hate you to see the price of fuel in the Highlands. Of course these price increases are due to transport increases and they're the only stations around and on main car transport links, so they have the sellers market.
I think what you're forgetting is that each station is run as a seperate store, with it's own profit and loss. Yes sometimes national stores will prop up failing stores, but mostly they have their own profits to think about. If it costs more to transport the fuel down a 10mile country track then the station has to increase costs to cover this.
You're letter of complaint should be a letter of society for accepting these price difference and not to the company. If you feel like writing to the CEO then write to every other business owner and chamber of commerce in the UK please. That's the only thing that will help with price variations.
In the meantime if you're willing to travel to save a thrupence then I suggest signing up to this website:
http://www.petrolprices.com/
Five petrol station's "locally" within 2.36miles- most I wouldn't even consider unless seriously cheaper, with three price variations. Shocking.
And here's a serious question? Are you sure both are 100% Tesco and one isn't a Tesco/Esso hybrid as this affects the pricing.