Got a slightly confusing radiator problem. New rads have been fitted downstairs but the small bedroom one is causing problems. Each rad can heat up independently while all others are closed off and then by opening each other rad just a tiny bit lets them all heat up apart from the living room one. If the small bedroom one is completely off then the living room is too hot to touch but as soon as it's even slightly opened up the living room one goes cold. The boiler is a Baxi with the tank in the loft, there is no air left in the system and the water came out clear when drained to replace the downstairs rads. Any suggestions on what to try next please?