The price of diesel at the pump always falls behind on oil prices because the price of oil today is the price "out of the ground", it takes some weeks (probably 4 � 6) to be turned into diesel and get to the filling station.
A few other factors should be taken into account:
1. Modern diesel cars are 20-40% more fuel efficient than petrol cars, and modern regulations mean they are polluting less than they ever did. So people are seeing more benefit and thus buying diesel cars. The demand is increasing. Also in particularly bad winters the demand for distillate heating oil is high, so this affects the price of diesel also.
2. About 9% of a barrel of oil is used to produce diesel. When demand goes up, refineries have to change their refining and distribution system which may take several weeks to implement.
3. Of that 106ppl about 72p goes on fuel duty, which is a constant and generally changes annually. Between 1993 and 1999 there was a rapid increase with duties on fuel increasing by 3% above inflation. This was due to a major change in petrol taxation in 1993 when the Conservatives introduced the Fuel Price 'escalator'. This was a way of the government making money and also to help protect the environment by discouraging people from using their cars.
One could argue that these forces will affect the slow barrel2pump price fluctuations when prices decrease. Yet, seems to occur much faster when there is an increase.