OK. Did a quick sprint out...
Ambient- 13.6C
Nano coolant temp 11.2C
Temp gauge 1st of the blue lines
Ignition on, but engine not started.
I'll do another dataset with engine startup from cold after breakfast.

Well, definitely something still strange going on with ECM coolant temp readings.
Did a run from cold start at home to workshop where I let it idle for a bit. No overheating, gauge didn't go past just below 1/2, IR temps around 90:

After tinkering with LPG for a bit, took it for a run around the lanes. No overheating, gauge didn't go past just below 1/2, IR temps around 90 again.

It's not smelling hot, bubbling in cooling system when turned off, ejecting steam or fluids. Top hose is as firm as when I pressure test cooling system to about 12 PSI, so I'm almost (!) confident I'm not cooking the motor.
Anyone actually had their cooling system up to 105C and seen where temp gauge points?
Can't tell from schematics whether overheat warning lamp is driven from gauge/ BECM data or ECM data. Either way, it doesn't come on.

On my SE, which due to the Britpart water pump being completely sh*gged after less than 2000 miles, yes. That's how I know how damped the dash gauge is because it got up to 105 degrees and the gauge still sat in the middle, it was only after it rose above that the gauge started to rise. The gauge sits in the middle at anything between 85 and 105, only hitting the red when it gets to 110 degrees, but by then you know that it is overheating from the other symptoms......

More temperature sensor anomalies.
Thought I'd get the LPG up and running this weekend- it was kicking in and then turning off with an alarm beep almost straight away. Plugged in the laptop. Reducer temperature -40C !
Checked thermistor in sensor. Open circuit. That explains that then. Must have broken a wire in there while using the sensor as a bleed point for the cooling system. Irritating that I'd fitted a Superseal connector in the cable replacing the direct hard wire to ECU to tidy things up when I was doing the heads.
I was anticipating using reducer temp to cross check the other coolant sensor. That'll have to wait until tomorrow now.

FWIW, in VAG circles its super common for aftermarket sensors to act wonky and its one of the items thats a firm "buy genuine" (along with MAF sensors!)

Hopefully your new new sensor is ok though!

Yep- I used to move in VAG circles. My Golf VR6 was fussy about wanting genuine for anything that measured anything :)
The New New engine temp sensor was genuine. Still getting weird data from the engine ECM side though. As for the LPG reducer temp sensor, I'm not sure that there is such a thing as a genuine Zeta sensor as I'm sure they just bought in a generic one. Just by coincidence it is badged as a Zeta sensor though.

Actual temperature isn't important with the LPG sensor, as long as it is showing that the reducer is warm enough that the coolant isn't going to freeze. You could even short it out so it thinks it is very hot and manually change over once the engine has had a few minutes to warm up. I must admit, I've always thought that a variable resistance can't be that accurate anyway, a few Ohms one way or another is going to be interpreted as a difference of a few degrees. My intake air temperatures always shows about 10 degrees lower than ambient on the Nano and I can't believe there's that much chill factor.

Oh yes, I also confirmed the heavily compressed temperature gauge yesterday. Spent the day at Woburn Safari Park and while sitting idling in a very long queue while monkeys put dirty footprints on my, and everyone else's, bonnet and roof, watching an Astra start to get very hot, I looked at my temperature gauge. Instead of the usual 12 o'clock, it was showing about 1 minute past, nowhere near the red yet but heading in that direction and higher than normal. Whipped the Nano out of the pocket on the back of the passenger seat and plugged it in. Coolant temperature showing as a steady 104 degrees. Considering it had been idling for well over an hour by then (following an 80 mph thrash to get there), that seemed about right. It stayed at between 102 and 104 for the next half hour or so until we started moving and it dropped down to 96 degrees as soon as we got moving by which time the gauge was back at 12 o'clock. Out of interest, I started it from cold this morning with the Nano plugged in and it was at 12 o'clock as soon as it reached 85 degrees. So anything between 85 and 102 degrees is shown as normal and it only starts to rise when it gets above that.

Thanks Gilbertd- I did stick a 10k pot (left over from my blend motor repairs) in place of the LPG temp sensor, just to confirm that it solved the non-switching problem, as I had hacked out several metres of superfluous cable from the LPG system when I was doing the heads.
At least you came away from Woburn with all of your wipers and mirrors...