Interesting Wammers thanks,
I understand how the thermo works with the system. Hadn't realised that the water pump could 'mill' as you put it, not having taken one apart....yet. The water can still circulate through the heads and the heater matrix whilst the thermo is closed though, right ?
How is air expelled from the system ? If air goes past the thermo on my RR, it looks like it would end up in the top hose, which appears to be the highest point. Presume the water pump will push it round a bit on higher revs but where can it get out. I read something about making sure one of the smaller hoses is water free when you refill, is that to allow air out ?