"To be fair, as far as I can tell modern air-co units take less than 10%."
That was a finger in the air figure, kind of aimed at the Prius, which doesn't have many geegees under the bonnet to start with!
"It's also worth remembering that you can lose 10% in fuel economy while driving with the windows down."
A valid point, although I think you'd need all the windows open to get near 10%, same goes for roof racks.
"You also mention using veg oil etc. Apparently you need to be careful as this can eat the seals in some cars, plus there's the issue of HM Revenue........."
I don't think it eats the seals so much as shifts all the old gunk that has collected there, which has probably been preventing a leak in the first place.
No tax issue with HM Gov any more, personal use, less than 2500 litre (I think) and it doesn't cost you a penny.
"Finally, according to Ford diesel cars are not as efficient for short journeys. I've certainly seen that for myself. Compared to a petrol car the diesel takes an age to get up to temperature and hates short journeys."
Indeed, but for short journeys a bus, your legs or a bicycle would be even better.