Well there's a reason for that

I did have to smile at the 'evidence' that diesel cars were actually more expensive to run that petrol cars. because the actual reason why is in the article.
Mail.

Great diesel myth: They DON'T save you money and petrol models 'are more economical for most makes of car'

• Can take up to 14 years before diesel cars save the average driver money
• Fuel can be almost 6p a litre more expensive than unleaded petrol
• Findings come as diesels make up over half of new car market for first time.

Diesel cars’ reputation for saving drivers money has been shattered by research showing they are often more expensive than petrol models.
It concludes that ‘diesels are no longer the default option for frugal motoring’.
While diesel engines may deliver more miles per gallon, it can take up to 14 years before they save the average driver any money.
This is because of the higher cost of diesel cars and of the fuel, which can be almost 6p a litre more expensive than unleaded petrol.

Yes, the reason that diesel cars take so long to pay back their costs is simply that the greedy government actually taxes diesel at a much higher rate than it does petrol. The article also seems to have missed the evidence that a diesel engine will last up to 3 times as long as a petrol engine if it's kept serviced, simply because the build of the engine requires a far higher degree of  internal strength owing to the pressures and torque of the engines. basically diesels are simply better built and this is reflected in the price of a new vehicle, though where diesels actually come into their own is in the second hand market where you can pick up the equivalent diesel model at the same price as a petrol model and expect to get a far higher mpg plus a car that will simply keep going far longer than its petrol driven cousin.
If the greedy sods in government were ever to reduce the fuel duty on diesel to the same as petrol then the payback on diesels would increase significantly. As it is, the only reasons it isn't is directly due to government interference.