WZ Ecoboost not getting up to temp

[Ben228]

Had this problem for a little while now.

Essentially, the car (1.0 Turbo Fiesta Sport) is not getting up to operating temp (generally 0 bars on the temp indication on the dash), also I'll only get cool-ish air through the heater.

Occasionally when it's warm/in traffic/driving up some hills I'll see 2/3/4 bars of temp (and the heater will blow warm-hot air), as soon as the load is off the engine it'll quickly lose the bars (and the heater will slowly go back to blowing ambient air).

For me, the obvious answer was one (or both) of the thermostats is stuck open.
Ordered (both genuine) a block thermostat and main thermostat.
Replaced them both (tested old ones which turned out to be serviceable, but given I had new ones replaced them anyway).
Problem is still exactly the same. Did not fix.

I bench tested the Coolant Temp Sensor just in case. At cold I had a resistance of 30K Ohms, and the resistance decreased as I applied heat, so I THINK the Coolant Temp Sensor is fine?
But as far as I know, it's the only other thing in the system I haven't replaced.

I'm out of ideas now.
Any suggestions?

Thanks in advance!

EDIT: I should also add, no codes are being thrown either.