The petrol prices go up when the tanks are near empty and waiting on a on a ship to off load the petrol. The prices only go down when the tanks are full and they need to get rid of it fast and another ship is due in port within a day or so. The oil companies dont like to have ships waiting it cost them thousands and thousands of dollars a day to have a ship sitting there.
DP works for Shell so he kindly explained it to me.
But as for it being so high for us consumers its the petrol stations that decide. And they all follow one another on pricing the petrol.