Perfect thanks for your help, Joey setting the distance in the default window to 0 got rid of the line between the windows but still showed the frames thickness between the mulled windows.
Our windows are aluminium no casings fitted.
Our plasterboard fits into grooves in the window liners (Frames) and steps below is how I got to windows to look the way I wanted them (sorry sizes are metric)
Thanks all with your help
Regards Rob (New Zealand
Then mulled the three windows together
then set the casing as below