Aggregate Demand and Supply

A nice article looking at the nature of the fuel market: supermarkets are cutting prices, in response to a fall in wholesale prices. Thus, you could look at the ways in which falling costs affect the equilibrium price of petrol.

However, the article also notes that many commentators are quick to point out that the price of fuel could have fallen quicker - suggesting that the supermarkets have a degree of market power in the market - allowing you to consider whether it's an oligopoly and the possibility that it's best described by the kinked demand curve.