Well we seem to have solved the problem... hopefully!
We borrowed another monitor with an hdmi connection and plugged that in and... no flickering.
We tried the borrowed monitor with the VGA connection and... flickering!
It seems that there is a conflict/interference between the plasma hdmi connection and the monitors VGA connection.
An analogue/digital conflict?
With two hdmi connections there is no flickering.
Our monitor only has VGA... looks like we might need a new monitor