The initial problem was that connecting the TV directly to the HDMI port resulted in the TV becoming the primary screen and this could not be changed.
It must be something in the graphics card that...

Finally got it sorted.
Connecting both the monitor and the TV using the display port sockets allows me to select which should be the primary monitor.
TV is connected using a standard HDMI cable and...

Just a further thought.
I wonder what will happen if I connect the monitor to the graphics card using display ports.
Or, connect the TV to the graphics card with a display port to HDMI cable and...

I have a similar issue.
Graphics card NVidia GeForce GTX 1060 (connected via DVI)
HD TV (connected via HDMI )
Motherboard Gigabyte B360M DS3H

I have spoken at length with the PC manufacturer....