I did what you said.
The radiator started getting warm. I ran the tap for 5 minutes and it is still running cold. I never had problems getting hot water before the pressure dropped. I thought it just needed re-filling.
I would be interested to know what you think now.
Thanks for your help.
I have always had trouble re-filling the system when the pressure has dropped. The system has been off for about a week now, but I have just managed to get it topped up. I increased the pressure to 1.5 bar although the white key did not seem to do anything (?) as I can twist the small white...