Two days same speed ~50mph 37 miles - yesterday wet and 12C 68.9mpg -today warm and dry 17.5c 81mpg. The weather seems to have an affect.

Instantaneous mpg Ken? You mean the one that often says you're mpg is 99.9?

You need to retry using the average memory 1 and reset whilst at a steady speed and run for a few miles with air con, and reset and repeat without.

You won't see much difference

Simple experiment for a warm day. I can't do this because my cars don't have real-time fuel readouts.

Idle the car with the air-con off and engine warm. Observe fuel readout in litres/hour

Open doors, turn air-con on full cold and full fan. Observe fuel readout in litres/hour

Unless you are in a desert somewhere, the AC will start chopping in/out once the evaporator reaches it's set temperature (2C on one of my cars). When it chops out the A/C is not using any crank power, when the evaporator warms up it'll chop back in again and the observed fuel consumption will climb.