grandad wrote:
Every filling station in my town has exactly the same price. £91.9 for petrol and £105.9 for diesel.

In any other unregulated business I am sure this would be seen as a cartel!
I think the theory is that if there are very few suppliers (but not quite a monopoly) and prices are easily comparable then they tend to charge the same without actually agreeing to.
For example, two filling stations side by side. If one drops their price then the other knows it'll lose a lot of custom thus it will drop its price too, so they both end up with the same custom but with less profit, so what's the point of lowering prices?
On the other hand, if one raises its prices then they know they'll lose a lot of business, which might even wipe out the extra profit margin.
So it's a kind of can't win situation, so they both end up charging the same.