Well, 2 things:
1. The beta upgrade via Terminal chewed away for about 90 minutes and then stalled: assuming
that was that I rebooted the system and the update had NOT been applied.
2. When I restarted I thought my system was bricked . . .
Tried Command-R, S, option and so on: when my wife came into the room and 'admired' my
new SATA Hard disk docking system (Oh, No: you've been spending money gratuitously again.).
I disconnected the thing so she could look at it and my Mac Mini booted up perfectly OK. My wife
said that was divine punishment for buying the docking station: possibly.
I have noticed that if I try to boot up with a powered USB hub connected to one of my Thunderbolt - HDMI
"thingies", the monitor that is connected to that thingy doesn't display anything: as soon as I yank out the
power cord from the hub the monitor comes to life: I can then reconnect the power cord.