Yes, made some progress over the weekend. I've been holding off posting results as I wanted a few cold starts under my belt to know it's ok.
I started the car Sat morning with the laptop connected. As sods law dictates, the error did not occur that time. I checked for codes anyway, and there were none stored.
I got a 100Amp battery drop tester, connected it up to the battery and it read 12.0V. I then started the car. The initial Volt drop when cranking was down to 8V.
After the car ran for a minute, I shut it off and the Volt meter was then sitting at about 11.6V. I did the 100A drop test which dropped down to 9.5V.
I took the battery inside and charged it.
At full charge, battery was reading 12.6V. 100A drop test dropped that to 10V, recovering to 12.0V.
I wasn't happy with that, so I went and bought a new battery.
New battery drop test went down to 11V and recovered back up to 12.6V.
New battery in the car dropped to minimum of 9.8V when cranking.
I had not noticed any other battery related type problems and this car is my daily driver (about 30miles a day commute).
But the new battery does somehow "feel" stronger when cranking and the gearbox error has not happened since (only been a few days, but there was frost this morning).
Assuming the problem does not return, I think that initial drop to 8V when cranking was too low to keep the TCU alive / happy (it has emotions now).
That makes sense to me with no codes stored. If it was the TCU dropping out due to momentary low Voltage, it might not have a chance to store codes.
Like expecting a Windows log to log a power cut I suppose.
This would mean, any error such as this, it's worth checking battery Voltage when cranking. General internet consensus seems to be that trouble occurs with car electrical systems if Voltage ever drops below 9V (seems to be the magic number).
If anyone is really interested, I video'd myself doing the battery tests (mostly as I wanted to capture the momentary Volt drop easier, but then I got carried away and put it on Youtube in case I wanted to refer to it...
https://www.youtube.com/watch?v=VG-2...ature=youtu.be