"Diesel gennys run about 0.4l per kWhr, so need about 16 litres...

"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