This seems to do the trick:
G18 G21 G40 G49 G64 G80 G90 G94
G0 X0 Z0
G1 X20 F200
G1 Z-10
G2 X20 Z-50 R100
G1 Z-30
This was done by substituting my values into one of the routines at the end of the Arc Motion document, also same as Hood's suggestion. However it only works with the 'reversed arcs in front toolpost' box unticked in Config. This is a bit of a pain because I have other arc routines written with the wizard and I think they won't work now. However, it would be easy enough to change the config when I need to use those routines.
The important thing is that having got something to work, I can play about with the other sample programs knowing that I have a fallback position.
So, thanks to Hood and Rich, not for the first time!
BTW Rich, I would buy a lathe CAM package but I actually work in wood (making bagpipes) and much cutting is done using a parting off tool, using the sides of the tool in both Z+ and Z- directions. This would make any metalworker wince and is not catered for in any normal CAM package!
Cheers
Richard