Correct and a 15mm gate valve that I did not know (shelves full of towels etc) between the header tank to the CH pipework (joining into the CH pipework just below the air separator) that I did know about.
It looks like the valve was not fully open (and has probably been like that for 20 years) allowing crud and to fill the 15mm feeder pipe.
New gate valve and a bucket of water through the header tank and all is well.
Thank you to you and everyone else that has helped.
Don't forget the inhibitor :-)
I did. We filled the CH circuit up that way and eventually it started adding more water to the already full CH header tank via the expansion pipe.
I now realise the logical conclusion at this point would have been to block the expansion pipe and try to fill the header tank via it's feed pipe (ie the 15mm gravity feed pipe that the tank uses to feed the CH pipework from) or in other words (Star Trek and Scotty anyone, just reverse the gravity laddie) to prove that this pipe was blocked.
Cheers for the help. Much appreciated.
I certainly did not. I have shit loads of the stuff as some new-builds I was working on had electric only heating. The bathroom and en suite had water filled towel rads with electric elements and they sent a full bottle of inhibitor with every house. One bottle did all 20 houses and I kept the rest:-)
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