TPS, Rich, many thanks for these suggestions, I'll follow them up. Are the other points I asked about just "features" then?
So, I'm rather pleased that my tool referencing system seems to work quite well. I can use a setting bar in the spindle which fits direct in the MT4 taper and has a carefully measured diameter spigot at the end, to measure the offsets on all my tools mounted on Dixon holders, having first homed the X axis using a "touch plate" mounted on the apron. So for the first time this afternoon, I went through the process of switching on, launching Mach, homing X, setting the tool to the end of a bar (using an OEM button script controlling the same referencing system), and taking a small cut on the bar to a target diameter of 15.900 mm. I've been carefull to measure the test bar diameter to micron indicated precision on my Mitutoyo digital micrometer and record all the offsets etc to 4 decimal places (though I'm not pretending that the system is going to have real micron accuracy). Anyway, the measured diameter of the test piece after that measured with the Mit was 15.900 mm indicated. Only one trial of course, but quite gratifying, given the number of things that had to all go right for the system to work.
Now I have found another problem! Up to now I've always ignored tool offsets and simply referenced the tool to the workpiece, setting the pgm coords in the X and Z DROs to the bar end (zero) and stock diameter. Most of my turning has used wizards. Now I'm trying to use the tool offsets and homing the machine, and I find some strange behaviour. If for example I load some wizard-generated code and run it, first checking the tool position is correct and having selected the tool number manually (as Wizards don't have that facility), when it gets to the M30 at the end of the code and rewinds the offset is cancelled, though the same tool number is still displayed!!
A bit of experimentation shows that M30 has this effect generally on its own - for example select a tool with a programmed offset, so that the correct pgm coord for say X is displayed, then typing M30 in the MDI - after a short pause the displayed X position reverts to the "tool 0" coord.
I also find that typing "T<n>" as a code, either in the program or MDI, does not seem to change the tool number.
So at the moment, either it seems I have to make sure to manually select the tool before running the code, and after any M30 or rewind operations. Is this expected behaviour please?