@going,
since my last message I continued investigating and I have managed to get it to work. I had tried the "&" suggestion but that did not work in my case probably for the following reason.
bCNC uses graphics through Tkinter which in itself requires a GUI to be already invoked. It seems rc.local, and I also suspect crontab, are started before the GUI is invoked so I think this is why my program did not start. Of course it would have helped if I had been given some error message!
The solution was to create a .desktop file at ~/.config/autostart/ . The bCNC.desktop file needs to contain the EXEC line formatted as follows:
Exec=/usr/bin/python3 /home/xxx/.local/bin/bCNC
Note that the full path is required and, strangely, the "-m" option needs to be removed.
regards