Shell Fuel Prices Rip off

I noticed today that the prices at 2 Shell garages that I pass on the way to and from work are vastly different. Neither are on a motorway and they're both on similar A class dual carriageway roads although one is a faster road than the other.

The difference in fuel prices between the two is incredible. On the same day one is selling diesel at 137.9p per litre, the other is 125.9p. That's a massive 12p per litre difference between 2 outlets of the same brand in the same local area.

How can Shell possibly justify that difference?