Yes, the house is currently warm and will stay that way until a point where the external temperature drops. Sometime after this, the house will become cold because the flow temperature will not increase in accordance with the WDC. In this circumstance, occupants of any status will most likely cease to be content 
Some more unexpected behavior today. Unit is set to stop heating at 15 degrees outside. It’s now reading 18 outside and it’s still chugging away happily. Previously, it has always turned off immediately at 15. I’m assuming the two issues are linked and something is preventing responses to external temperature change. I’ve left the door open (so the thermostat doesn’t turn it off) to see what, if anything, happens.
Been racking my brains about this and the only thing I can think of relates to the external temperature sensor on the north side of the house. Does anyone know anything about how the external ambient sensor averaging works? The sensor reading fluctuates rapidly with no averaging enabled but the shortest averaging period is 12 hrs. Does this create a lag? If this is the case, it makes sense but I don’t understand why it’s only become noticable recently.
I’ve moved our external temperature sensor from a very sheltered north (brick and tiled roof on 3 faces around it) to under a thick wooden soffit on the south side.
Now instead of the temp sensor being about 4 to 5°C lower than ambient and it entirely missing the amount of solar gain we get when sunny, it actual tracks what’s going on much better!
It’s still in the shade as it were, but it feels what the house feels.
When there’s no sun, it’s also showing the correct cooler temperature.
Your problem sounds more like it’s stuck not reading the moving temp.
The sensors are a 2 bit part. Simply a two wire thermocouple / 4 screw connection block to the cable.
The cover just pops off. One central screw secures the base.
Really easy to replace / move.
Our external sensor is set to Averaging time: No!
No offset either.