I think that you may have misunderstood the point. You should have *two*
header tanks - a big one and a little one.


Bingo! You read my tiny mind - ballcock in the little one was floating
in the air! I'll keep an eye on water heating working less efficiently
than before - I guess I might have some air in the system now - but
normal service has been resumed so I'm happy.

Thanks to everyone for their help - very much appreciated.