I wondered if I need to open the radiator bleed screws to aallow
filling, but if the pressure dropped as a result of bleeding the middle
level rad, that water would be replaced by the level dropping one the
top floor creating a degree of vacuum which should get filled when the
valves open.

Actually, what happens is that you have a pressure vessel half full of
water. When you bleed the rad, the water comes out of here until the
pressure drops to zero. When you refill, the additional water compresses
into the vessel and the pressure rises.

There has to be something preventing the water getting through. The valve
may still be damaged, even if the handle turns. Alternatively, if you have
really low mains pressure, you may not be able to refill. (Indeed, if you
have a missing check valve, then you could be filling the mains with your
filthy boiler water!)

Christian.