It only occurs when the heating is on.
I do have the valves with the big numbers on as you put it on all apart from 1 rad.
I do see the gauge going up for the pressure when i top it up.
Thanks for you help, I am away for the weekend so cannot try what you have suggested.
In the past...
I moved into a new build in Sept 2007 and ever since I have had problems with my boiler.
It makes a banging sound then an error message of L9 appears, I reset the boiler and I get alternating 0 and 3 dashed lines and the boiler is roaring like a jet plane (error code is not in the manual)...