Yeah, I eventually found it - but importing the yymps lists doc html file in included file.
Anyway - after finding window_set_topmost(1); it don't actually do what I was after using something like....
window_set_rectangle(0, 0, 5120, 1600);
window_set_fullscreen(false);
window_set_position(0, 0); // These can be negative in some situations
window_set_showborder(false);
Taskbar still shows - think I'll go try it with a 'proper' language - must be possible as several games non-GM appear to do it without using exclusive full screen