Series 3 diesel temp gauge barely rising ?

Hi everyone, first time posting. My 1975 series 3 swb Diesel engine temperature doesn’t seem to go up, even after a good run, the needle is barely into the white section on the gauge. I changed out the thermostat for an 82c one back in late September and replaced the antifreeze and the gauge went up to normal operating temps. I haven’t driven it since then and hopped in it the other day and the temp gauge is back to its old tricks again. Could this be just because the ambient temperatures are lower now? Or could there be something else at play? I should also note that I drove it with one of those radiator cover flaps over the grille and this also made no difference.
Thanks.

It could be the sender, the gauge or the voltage regulator that are [playing up. The only way to be sure is to test each component individually. Of course you can't rule out the thermostat, just because its 'fairly new' doesn't mean its not packed up. I'd test that first. How hot is the top hose/top of the rad?

Only one way to find out

Yeah temp at the thermostat housing averaging around 80 after a good hours run at “fast speeds”, managed to get it displaying normal temps on the gauge, but as soon as I switch the heater on it drops back down to the bottom of the gauge in no time. Tested the Thermostat and all works correctly, I think the cold weather just means it takes bloody ages to heat up? Main thing is it’s not overheating. Come summertime if it still does it, then maybe there is something wrong with the sender, gauge or something else.