I had a problem similar to this in one rented house, the difference being that the upstairs radiators were scorching hot but downstairs were cold. Turned out to be a bust pump.
its probably your waterpump you can ease back the round cover of pump and you will see a notched screw turn it with a screwdriver that should free it,or else your rads are badly airlocked also feel the inlet pipe to rads if hot you know they are airlocked,so bleed them