In message ,
newshound writes
On 06/10/2019 13:00, ARW wrote:
The water to the header tank was turned off and the system drained down.
Swapped the mid position vale and it will now not fill back up.
The odd thing is the header tank did not drain off - it was still
full when I went to turn the water on.
Suggestions?
-- Adam

Airlock.

No CH when I was a kid, but a routine at home every time a hot water
tap washer had to be changed was to connect a hose between hot and cold
water taps in the bathroom, and turn both on.

I don't suppose that was a cottage in Southwold?:-)

--
Tim Lamb