Solution (of sorts) found.   I have replaced the HDMI cable to the second screen with a display port cable, and it is now functioning normally.    No idea why it wasn't working before, but at least I can get back to work now.
I've got an XPS 13 and a Dell Thunderbolt 19 docking station.   I have two monitors linked to the docking station, one through Thunderbolt and the other through HDMI. The laptop is only extending the display onto the monitor connected through the thunderbolt cable.   Hitting 'Detect' on the Display setting does't find the additional screen.    However, when I turn off the monitor connected through Thunderbolt then the computer automatically finds and displays on the HDMI screen.   I then switch the other monitor back on, and the display reverts back to that screen and the HDMI says no signal detected. Any ideas?                                                                                                                                       
Can't find second external monitor