Thanks, Steve, but the knob works freely.
Using a torch I could see the mechanism through the grille. There's a thin flat springy lever that pushes open the flap against gravity. It's operated by a cam from the control knob. The lever on each of our three heaters was far too high, and wouldn't even touch the flap. I opened the front panel (two screws below and a lip on top) and adjusted it. There's an adjusting screw, but the lever was far beyond its range, and needed bending with pliers. Now it's fine.
I'll fix the other two once we've got over a frozen water supply and a burst pipe.
Gordon
December 2010