Depending on the crude oil itself there is a limit to how much low sulphur diesel can be refined from each barrel of crude, unless new types of multi million pound refineries are built. Unfortunately, worldwide demand is relatively greater for the amount of diesel that can be refined from crude than it is for petrol. It's used for trucking, shipping, railways, heating, farming, etc. So really, everyone who uses it in private cars is putting up the cost of diesel for more essential uses, which in turn puts up the price of everything. Europe has more diesel then the UK because it can store more, which is why diesel is dearer in the UK than in Europe. But in countries like Thailand diesel is around 20% dearer than petrol, and they don't need it for heating because it never gets cold enough. The duty is the same 52 .35p + VAT that it is for petrol, + vat on the fuel itself.