Hi,
Just to say a quick thanks, I have sorted it, and embarrassingly easily!
Because I have the old VGA screen I have not been using the new screen in safe mode to save having to switch it out of safe mode every time I want to see if it is working. I decided to follow your advice to the letter and changed to safe mode to change the resolution, I went into the monitor on screen menu to change the brightness and there was an option Analogue or Digital and it was set to analogue! Switch to digital, reboot and all working perfectly.