I seem to have a challenge in refilling the CH system after draining it
down. My method is to close all the air valves etc., turn on the water, wait
until it fills. Then I bleed the air from the radiators and turn on the
pump/heating. At this point, I hear lots of glugging in the boiler part of
the system, as if it was previously closed off to the water until the
pump/heating is turned on.

I have turned the screw cap in the centre of the pump to then bleed the
system again, and then bleed all the radiators again to get rid of residual
air in them.

However I'm not convinced I'm getting all the air out of the system near the
boiler as I get some overheating issues on the boiler (it goes away again
when I drain and refill the system, pointing to an airlock).

Am I missing something? Is there a way to open the valve in that area of the
system to fill it up before turning it on?

Thoughts on technique or other appreciated
Thanks