(yes I know it's 35 years old and needs replacing - I'm just trying to nurse
it through until spring!)

The boiler is overheating (radiators too hot, HW scalding and boiler
creaking a bit). The room and HW cylinder thermostats are working ok, so I
thought the problem was the boiler thermostat. Replaced it with a new one
last week but problem still persists. If I turn the boiler thermostat down
to zero it clicks off and the boiler shuts down, but putting it to any other
setting from 1 to 6 the boiler just keeps going until the room/cylinder stat
shuts it down. Turning the knob through it's range there's no feel/sound of
it switching as I'm sure the old one used to do. I suppose the new
thermostat could be faulty - any way of testing it?

I have recently flushed the system in anticipation of getting a new boiler,
but then plans delayed so I refilled with new inhibitor. When draining the
system I found both the drain tap on the lowest rad and the one on the
boiler frozen/blocked, so I fitted a new one on the rad but didn't bother
with the boiler one (at the time though at new boiler was imminent). Could
there be an air lock or sludge in the boiler which is somehow affecting the
thermostat but still allowing the boiler to produce (very) hot water?

Grateful for any advice.
David
(remove _notme from email address)