That seems to have fixed it.
Many thanks for the help, Dave/gas4you. Guys like you who generously give up their time and expertise to help the clueless -and save us a small fortune in the process- are diamonds.
That is excellent news.
The system fills form a large tank in the attic. It's an 30 year old system in a ground floor flat of a house conversion.
As I said, the system was working well for years until the fan and circuit board died on the boiler. I think the engineer who serviced the boiler...
I tested the gas from the radiator but there was no pop. Wouldn't air that had been through the pipes smell slightly stale anyway?
As I previously posted, the rad closest to the boiler was literally filling up with air -it took over a minute to release the air if I left the CH on for a few...
Thank you for your responses.
Both the boiler and the radiator are on the ground floor.
Hmmm -the engineer didn't go near the loft, but...
He may well have done -is it safe to turn it back down again -and would it be clockwise up /anticlockwise -down?
Just had my Halstead Best boiler serviced -fan and circuit board replaced- and it is working nicely. The problem is that a huge amount of air is collecting in the radiator nearest the boiler. I called the Corgi engineer who carried out the service and he suggested corrosion was creating gasses...