When Cadburys had to withdraw some of their confectionery items some months ago, did their rivals increase the price of their similar products? Of course they didn't. The petrol suppliers are not there to supply their product at a reasonable cost. They may go through the motions of independently pricing the petrol but it isn't a genuine approach to the market. The motorist is a captive market for these firms so they collaborate to maintain a higher price. Isn't there some government department who can and will show more supervision over what is going on?
I agree with all you say but I was reacting to news that filling stations or their companies immediately increased prices following the discovery of silicone at some other filling stations. That was where the analogy of the Cadbury bars came in.