"It's got the most energy of all fuels. Hence its efficiency and lower CO2 output."
You've missed the point, AC#1.
DERV use may result in lower CO2 emissions, but all the other nasty pollutants have increased as a result and people are now realising that choking themselves may not be the best course of action.
"In France Diesel is a lot cheaper than petrol. So can you really see the French government lowering the cost of petrol?"
Why would you think they'd reduce tax on petrol use, AC#2 ?!
DERV prices have been rising steadily over the last 5 years in France as gouv.fr has been been putting more on gazole, not decreasing tax on essence.