Happy to say that folk over on Cloudy Nights helped me to resolve this issue.
Particularly Michael Swanson who runs the Nexstar resource site,
https://www.nexstarsite.com/
I spent $30 on ebay getting an old desktop PC with a serial port, to avoid using a USB adapter.
Plugged in the SLT mount and ran the MCupdate program with no hassles at all.
Very happy with the outcome, and amazed at how fast Windows 7 operated!
Have also installed a Celestron Skyportal Wifi unit, and am now running the scope from Sky Safari.
