Going down the avenue of small cars qualifying for £35 / free road tax, you again end up with petrol coming out very well.
Here are the basic facts to do with diesels:
Diesel engines are heavier than petrol engines.
Diesel engines are noisier than petrol engines.
Diesel engines use less fuel to produce the same output as a petrol engine.
Diesel engines are considerably more expensive to manufacture due to emissions kit.
The result of this is that for small cars, diesel engines are a bit of a waste of time – the extra weight of the engine is a significant proportion of the car's total weight, as is the extra sound deadening, and the car costs more to make in the first place. The extra weight takes a chunk out of the fuel economy, so it really becomes a waste of time to have a diesel.
The place where a diesel engine comes into its own is for the bloke buying a new, largish car and expecting to do a lot of miles. You can then get out of the really punitive company car & road tax bands, save on fuel, but still have similar same performance in a straight line.