In reply to Anonymous: I think a key sentence in Brian Wilson's article is where he compares the price differential of Coca Cola vs fuel:
"Tesco’s profit margin in Stornoway may be a fraction lower due to transport costs but it is a tiny difference which they can well absorb."
As Neil pointed out - and as evidenced by the number of non-supermarket petrol stations closing down all over the country - profit margins on fuel are very small (in fact I'm pretty sure that a fair proportion of supermarket petrol stations operate at a loss, simply to draw customers to the shop). I think it's a safe assumption that the profit margin on Coke is much better (ie the cheap, possibly loss-making fuel is indeed acting as a loss-leader for the more profitable sugary brown fizz). So even assuming that the comparison Brian makes is actually valid, it's quite possible that the additional cost of getting a litre of fuel and a litre of Coke to Stornoway can be enough to completely wreck any kind of profit margin on the fuel, while still allowing a profit to be made on the Coke.
Bear in mind also that Tesco's pricing for Coke is transparent UK-wide - you just go to their web site and there it is. The same is not true for their fuel pricing. On that basis, it could be argued that people who buy Coke from Tescos in London are actually subsidising Tesco's Coke customers in Stornoway...
As See You Next Wednesday said, if there's profiteering going on then you, or someone, should be able to win market share from the established suppliers by offering more competitive pricing. (In fact I think that's one reason why Gleaner started to move into forecourt fuel sales.) Of course there's another problem in the Highlands and Islands, because fuel outlets tend to be much farther apart than in more urbanised areas. So even if you nearest Gleaner station is a few pence cheaper than your local Tesco, if you have to drive 30 miles to get there then the cost of the fuel you use to drive there wipes out any cost saving at the pump.