Now when coming out of standby, the screen power light comes on, the PC's fan & hard drive come on, but nothing on the screen.
However, if we move the mouse while it is powering up, the Welcome screen will appear, but when clicking on a user, the screen goes black again. It will respond to the keyboard, so 'Window Start Button' + L will bring up the welcome screen again.
The only way round this is a reboot.
I've installed the latest video drivers but it made no difference.