I decided to see how things work using an old laptop, just to make sure that the problems were not potentially software related. With the Latest Mint Cinnamon, Kstars etc, everything works perfectly, connects first time and has no issues I can see. I can connect to this and run everything from a windows pc, and also via a VNC connection to the laptop.
I have upgraded the firmware on the Pi to no avail. A search shows that the problems I have had with USB 3 are not unusual, not common, but by no means unknown. I will try one more option and set up ubuntukstars on the Pi over the weekend and see if that makes any difference.