Hugues, thanks for all your help.

First off you don't need to set the display screen to 800x600, you can revert to a native resolution (1920x1080 for me) on the desktop BUT you must apply 800x600 to the projector or "VGA Display"
I'm scanning at the moment....( OK I've moved the scanner fiddling with the connector) .. so it's not good.

first_miscalibrated_scan.jpg

It was a bad connection on the cable at the scanner end. I just wiggled the connector and tried to tighten the screws some more at the VGA plug, and the projector went bright white.
Then I closed the program, and hey presto it didn't lock out.

So for anyone else ...the important thing is to set the "second display" aka the VGA display to 800x600 otherwise it won't display the right white light patterns/stripes etc

When I am working on stuff, I find it always the simple things that catch us out, but if it wasn't for your posts, I wouldn't have understood how this thing works!

Many thanks... and it's a Sunday
Mike