further to my heat loss

Ideally if you can get the two pipes off from the heater core then you can flush just the heater core. The pipes were stuck fast on mine and could only manage removing one. I then decided to flush the whole system which was a faff. Had to remove the thermostat to allow the pipework to flow freely.

My symptoms were, warm air to begin with and then it dropped off. If CC put on High then I'd get hot air but no intermediate. After flushing the coolant I no longer have a problem.