The message .com
from "Jon" contains these words:

Hi there, I have got a set of those bungs that you can use to bung the
feed, and stick the other in the overflow expansion.

My new house seems to have one of these combined feed / expansion
configurations for filling the CH system.

I have attempted just bunging that one pipe does not seem to have the
desired effect - is it possible with this type of configuration?
Where is the air coming in on this (sorry if this seems a daft
question).

No idea whether the combined configeration is different but I found when
I used the bungs for the first time thaty I had to drain a little water
out of a radiator bleed screw in order to establish the partial vacuum
at the top of the system.

--
Roger Chapman