So, I buy a new graphics card so the old ones get shunted into the old renderslaves in order of their power and age... and suddenly.... two PC will display a black screen on TightVNC unless a physical monitor is attached to the PC and the monitor switched on!
The solution came from here:
and thanks to IT Zap, whose two monitor solution also works for Tight VNC
1) Go to Control Panel and click on Power Options.
2) In the menu on the left-hand side click Choose what the power buttons do.
3) Click Change settings currently unavailable.
4) Under Shut-down settings, UNTICK the Turn on fast start-up box.
Thank you IT Zap... works a treat
Well it didn't work a treat exactly. The computer with the GTX970 continued to be problematic until I installed one of these
With these you have to make sure you set the display resolution to something HDMI compatable like 1366x768.
Interestingly I bought a GTX1060 ( which is incredible for eeVee for the cost and second hand definately gives best bang for buck rendering ). But with the GTX10 series cards it seems that unless the HDMI dummy is there, you will see windows but Blender is just a white window. This seems to be the GTX10 series does not turn on OpenGL unless there is a monitor present and Blender's interface is all drawn in OpenGL.