I don't understand this stuff so I'm just repeating bits of what others are suggesting and it looks like that seems to work - starting Ekos (with ttymount set as the serial port) seeks to not report issues and I can drive the mount (N/S/E/W - issues with everything "below the horizon" at the moment but GPS can't work indoors and still investigating cause ... but that's another issue).
If you/anybody thinks this wrong and wants me to try anything else I'm very happy to experiment further. I am getting a large number of timeout errors (but I think I got those on ttyUSB0 - but that's think so more playing around and experimenting and a separate issue).
On the assumption this is "the answer", is this thread enough to pass the info to others or does the tutorial need updating.