I have a theory...
In this hustle and bustle modern world, with a high cost of living and little time to actually relax, many people have started drinking more, eating rubbish, and driving diesel cars.
This could prove disastrous for the country. We are already regarded as the third coolest nation in the EU, behind the Italians and the Greeks. If this trend towards DERV burners continues, we may fall to being as uncool as Germany, or even Belgium.
Combine this with cheap, badly-made clothes imported from China and we might sink even further to become the least cool nation in the EU... that's right... even less cool than France.
So, they bumped up the duty on diesel. This should cause more people to drive proper cars, with engines that go "BRRRRRMMM!!!" instead of "CHUGGACHUGGACHUGGACHUGGA!!!". It is then only a matter of time before we rise up again to become the coolest nation in the EU.
Fact: London in the late 1960s was THE place to be. Yet few if any people drove diesel cars in London back then.
Fact: The sudden increase in diesel car ownership corresponds EXACTLY with a sudden increase in divorce rate AND childhood obesity.
Fact: Buses are diesel, and it has been well documented that bus drivers suffer from more cases of piles and low sperm count.