Ok
I had a plumber replace the tank who then talked me through bleeding the tanks next day when I called him with reference the problem.
He is now on holiday.
10 days ago one of our expansion tanks leaked badly in the night and was replaced the next day. At the time of the leak I turned off the water supply and turned on all the hot taps to empty the tank.
The system we have is a Santon unvented cylinder with 2 expansion vessels above. 1 has a...