I can't tell you what will be in the November budget, but if tax on diesel increases then it will put up inflation because everything in the shops is at some time transported by diesel truck. Whether VED will rise for diesel cars is another matter, but that would mean putting up tax for around half of all motorists and would not be a vote winner either. Diesel is in enough trouble anyway due to the high cost of maintaining diesel EGRs, DPFs, AdBlue heaters, etc, so unless you do a huge mileage, or need to tow, or need a big 4x4, or need a truck, you'd be nuts to buy a new diesel.