Thank both for the ideas.
I did try connecting on a Ubuntu computer that had never had any INDI libs installed before (or Kstars), and I got the same not defined in . error.
Also, the user is a member of 'dial out', and sudo did not change the errors.

Good news sort of, as I got things working, but the bad news is that I used a work around, and the cause and fix of the "error: Property CONNECTION is not defined in ." remains unknown.

Here is what I did to get connected:

This assumes that you have a fairly new (mine dates from 2019) SynScan EQ5 that has a USB port on its hand controller (not only serial ports). The old ones don't have the USB-B port, and this method would not be suitable for those.
Don't connect a PC USB to the autoguider serial port on the motor controller box (the one clamped to your tripod legs) or to the serial port on the hand controller directly (it won't fit in that port anyway) because it can damage your motors and hand controller (to do that you need a FTDI USB Serial Device USB to serial converter).

1. Intall kstars and INDI driver etc (in a terminal, to get one type CTRL+ALT+t)

2. Start the SynScan on the mount and run through the set-up, all of it through to the star alignment (using the hand set as usual)
3. Do a 1-star alignment (just fake it, as probably, like me, you have the mount in doors with no OTA on it, just for testing)
4. Connect USB on PC to the base of the SynScan hand-controller using a A-Male to B-Male USB cable (this is the one most USB printers use). I took mine off my printer to try this.
5. Start Kstars and do Tools>Ekos to set up the connection. Call it SynScan and use the defaults.
6. Click connect and everything should turn green.
7. Now you can tweak settings as you like (input OTA details, make sure mount and Kstars are in same time zone, etc).
8. To make Kstars effect a GOTO function just right click on a star or M81 or something and choose synscan> goto
9. Watch the red cross hairs home in on your target, the mount should slew to it as well (of course).

# Alternatively you can do (in a terminal)then use HNSKY or similar to control the mount.

I noticed that if I turn the SynScan off, the hand controller remains powered on because it is getting power from the PC USB. So far it has not exploded.

Thank you very much for the tips and advice.

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