The file with (I believe) all bugs ironed out is now available
After even more help from David I think any bugs affecting functionality have been ironed out and the file attached here should work.
I would be grateful if people with a Duet could test it - so far I have tested it on very simple files (so I could view the g-code it produced).
Thanks to @David the swarfer, who did 90% (99.9?) of the work this is a modification of Ooznest's post processor for the Duet controller using Ooznest's firmware that restores the rapid moves that Autodesk removed in their changes to the personal use licence. It is beta - use at your own risk, but the worst I can envisage is getting soft limit errors.
Please feedback if you find any problems with it.
This modification also allows use of multiple wcs (G54 - G59) and puts a G53 G0 Z90 line at the beginning and end of the file (4mm short of Z home for Duet users).