I'll have to give that a go.
I drove 550 miles yesterday in stop start traffic and I had nearly 1.5 litres of coolant in my bottle at the end of the journey, so maybe I'm loosing coolant each time I accelerate up to speed?
As the heater goes cool at speed I'm almost convinced of a blockage resulting in a reduction of flow through the heater above 60 mph. At 30 mph I have to turn the controls right down as it gets very hot!