My guess is also that insurers realise that diesels are no longer being produced in any numbers (manufacturers can appear virtuous by dropping diesels, has the great advantage of reducing overheads by reducing engine options). Also, despite offering policies based on age, car etc, there will be a baseline overall cost. My guess that is pretty important in determining how much the premiums are, and keeping shareholders happy. There is plenty of evidence to suggest that companies took advantage of high inflation rates to push their margins up, and I suspect that once they done it once, the temptation to carry on is irresistible. After all bigger margins, bigger profits, happier shareholders and bigger executive recompense packages, what's not to like? (think water companies). Not being political, all based on published information.