Funny stuff, water. Expands when above 3C, and also when below 3C, and of course even more when turning solid.
So the top of a frozen lake is floating ice, and the lowest level of water at the bottom of the lake is the densest at +3C. Fortunately this gives fish somewhere to survive, albeit somewhat chilly.
Coolant expands with increasing temperature too, but as the air in the header tank is also under pressure, it acts to push the coolant level down, hence my question. The complexities of the RR cooling system are such that I wanted to know what happens in real life, with all the forces and counter-forces present in the header tank..