Again , many thanks for the excellent writeup.

It is really excellent & helped alot.

I finally got some data which is close... but I have one issue.

I can deal with offsets and factors to allow for hardware variations, but my issue seems to be something a little more random.

With any fixed voltage being supplied to any of the ADC channels I find that the output to the TP dash or monitor is almost perfect - apart from 1 small but significant issue.
At random intervals the voltage will spike high or low by around 50% (or more).
The voltage is fixed and verified by multimeter.

The output is as follows:
free air log.jpg
The unused +channels are wired to earth.

Any ideas would be very welcome.

Cheers, Chris