"Diesel gennys run about 0.4l per kWhr, so need about 16 litres to get the 40kWhr or so per extra car."
Think large modern diesel generators , use around 0.25 to o.3 l per kwh so more in favour of diesel generators
"Will never run 2*75kw charge points on solar and or battery'
A high discharge battery can be used to push it up to 150KW , same scenario as what Tesla does in South Australia , pretty much the same logic as using an accumulator in a hydraulic circuit for high discharge