dt90

Hello there. I am looking for some help with my switch from my current setup of Drayton LP522 and Honeywell DT90 to Nest 3rd gen thermostat. I have followed prevoius posts and tried to replace lp522 with Nest HeatLink. Here is a look at my lp522 And here is a look at the hairy junction box...

I've just replaced my Honeywell dial thermostat with a dt90. Very simple like for like replacement. However, the DT90 is now constantly showing 19 degrees as the room temperature. It never varies regardless of whether I turn the heating on or off. Does it need calibrating somehow (I looked at...