1.4 tdi AHF Strange Coolant Temp Issue

[99problems]

I changed the coolant temp sensor from genuine to aftermarket (green one) about 2 years ago as needle was jumping 0-90-issue resolved.

1 year later I noticed coolant leak from the sensor, changed for a new (aftermarket)sensor as I couldn't get the o ring separately-seal fixed but now only going 70c on the gauge. Foolishly I had binned the old working one but I thought I just had a faulty sensor so got a new one under warranty, changed this still the same issue.

Was going to put a genuine temp sensor in place however I thought I would just change the thermostat too as it was old & maybe I had gotten unlucky for both issues to happen at same time, fitted new non genuine thermostat in (I know!) & the gauge is now going to 90c on the dash after say 7-10miles-Issue fixed I think.

The 1st long run I went (150miles) the 1st 20miles the gauge is stuck to 90c as it should, after that it drops down to 70 & even goes below so I am thinking the coolant temperature sensor is faulty however at the end of the journey I check the temperature of coolant in the expansion bottle using the Mrs cooking thermometer & its 67c.

Now when doing short journeys its not getting to 90c as it was when I had just changed the thermostat, the hottest i have gotten the coolant in the expansion bottle is 72c & I the range of the sensor is supposed to be 75-92c from what i can find online (so it sits at 90 on gauge). 

So is the thermostat stuck open & causing issues or do i have a faulty sensor? Can't find my infrared gun to check temperature of coolant at sender location which is helpful🤦‍♂️.

Before I go waste £65 on a genuine coolant sensor is there anything else it can be or have I missed something?

Car is still running fine & giving heat through vents. 

Cheers.