So this is what I find....
I don't think it is the guage. I can get it to rise and fall using the
heater fan on full roast setting. It could be the sensor as the temp does
seem erratic. On the first 20 miles the temp behaved absolutely normally.
I then stopped for a while and when I re-started the temp rose within 2
miles to near to red light warning level. Switching on the heater brought
it down again. Given that both the warning light and the guage indicate
overheating, then I think it is not a guage problem.
I don't know much about the newer cars, but on my GTV-6 the gauge and
warning light are on the same sensor.