Drivers have long seen petrol prices change every time that they pass the same petrol station. It turns out now that the price being set bares no relation to the price of oil. Instead, each petrol station has a random number generator built into the pumps that randomly determines the price of the petrol and diesel between a wide margin.
"We've seen some petrol stations vary their prices by ten pence a litre over the course of a week," said Deborah Sell, from OfFuel, the government's Fuel Watchdog. "Sometimes, it will change two or three times a day."
According to Sell, even knowing the price of oil will not help determine the actual price change.
"For example," she said, "You might know that the price of oil has dropped recently, but that doesn't mean that the next price change will be down. It depends on the random number generator."
The only item at a petrol forecourt that does not vary in price, is the charcoal briquettes. Even the crisps and chocolate will change in price depending on the time of day and how much they have.
The petrol giants have all denied using the approach, but the evidence does not back them up.
"A forecourt will buy fuel at a price," said Sell. "You would expect therefore that the fuel would be sold at a slight mark up until all that fuel has gone. However, the price will vary."
OfFuel believe that the service stations are setting the random number generator to produce fuel prices between one and ten pence higher than the cost of the fuel.
"Our advice," said Sell, "is that if you think the price is too high, to wait an hour, and see if it comes down."