Above and beyond with that response Graham -- thanks.
As it happens I am familiar with the concepts of tool offset though I haven't previously handled it in G-Code -- I recently made a small box with M. C. Escher Lizards and some Initials inset but that was on a Roland MCX-20 which takes .DXF files. The letters and Lizards had to be cut out using an outside offset and the box lid with an Inside offset.
Using G-Code is a new experience for me but the concept of tool positioning is not. There was no way you could second guess my past experience and your very good explanation is greatly appreciated.
How I hadn't noticed [ CV Mode ] is beyond me
but oddly enough - like so many options in Mach3 - when I click on it nothing seems to happen ie. the green light doesn't change. It may well be that I'm in some state that precludes a change. Notwithstanding that, you'll understand that I've done more testing and have been successful in 'drawing' the shape along with a second trace showing the offset. I also noticed that the feed-rate did change as the pencil approached each bend. From that I presume that CV is 'Off'.
I had a number of 'issues' which had to be addressed. Primarily to do with initial positioning of the second (offset) trace. My first attempt was to simply duplicate the code from N230 to N320
%
O0000 (Radius test)
N100 G17 G21 G54 G40 D0
N230 G00 X-100 Y50
N240 Z-32
N250 G01 Z-33.5 F250 (no tool Comp)
N255 S2000 M03
N260 Y-70 F3000
N270 G03 X-80 Y-90 I20 J0
N280 G01 X80
N290 G03 X90 Y-80 I0 J10
N300 G01 Y10
N310 G03 X50 Y50 I-40 J0
N320 G01 X-100
N330 G17 G21 G54
N335 (8Ø tool compensation)
N340 G00 X-104 Y54 Z-32
N350 G01 G42 P4 Z-33.6 F250
N360 X-100 Y-70 F3000
N370 G03 X-80 Y-90 I20 J0
N380 G01 X80
N390 G03 X90 Y-80 I0 J10
N400 G01 Y10
N410 G03 X50 Y50 I-40 J0
N420 G01 X-130
N530 M05
N540 G40
N550 X200 Y100 Z0
N560 M30
%
... but I soon found that I also needed most of N100 as well. I then found that the compensation didn't work correctly since I'd left N340 as X-100 Y50 and once I'd changed that to -104 and 54 line N360 needed X-100 (this wasn't needed in the first run because it was Modal).
I did try to insert G42 P4 at line 335 but that gave me other errors -- I still can't fathom why - or how to circumvent them, though I suspect that there is a way to account for a tool offset without manually specifying a new X and Y in line N340.