Airlock in the heater matrix or indeed matrices as mentioned earlier in the thread.
This will def show symptoms of overheating until cleared.
The black hand turn knob you mention sounds like excatly what you are looking for, you need to warm the car up, switch the heaters on full blast and loosen this screw. Wait until only water flows from the opening, you should hear the air being released as well.
You may have to top the coolant up when the engine is cold and repeat this procedure several times to fully bleed the system.
Is bucketing down with rain at the moment... gotting to try a good bleed of the circuit.
I believe the coolant has its own resistance properties so hope to add more coolant....