Heater not doing much heating

I've always found Jimny heaters to be pretty incredible, but recently mine has become rubbish. The fan works in all speeds and airflow is great but it get's luke-warm at best.
The control cable on the side of the HVAC unit is connected and operating properly.
It has a new thermostat installed correctly.
I've flushed the heater pipes out with a garden hose and they flowed fast and clear.
The water pump is in good condition.
It's not using any coolant or blowing bubbles in the rad.

Anything else I could look for?

I took the control knob off mine to change the bulbs and for about a year afterwards my heater was lukewarm, it seems I didn’t quite get the control cable in correct place.

When I took it apart again and double checked the cable position there was only a slight bit more cable and it made a bit of a difference, but not convinced it was as hot as it used to be.

The usual culprits are a sticking thermostat (open), sludge in the heater matrix or air trapped in the matrix, but you seem to have covered these.

Robin