Ross wrote:
That's what's puzzling in that the radiators feel very hot, but the
rooms don't. When I bleed the rad upstairs, the water is black, so it
would be great if having them flushed through would solve the problem.
The fact that the rads feel hot makes me doubtful. Thanks again for all
your input Andy - much appreciated.

If they are too hot to touch, and the house is still 18C, then you are
either in need of more radiators or more insulation.