GOT IT !The X & Y co-ordinates are [ RELATIVE ] (or incremental) not Absolute
That now makes sense, Thanks.
That is not what the CNC Cookbook tutorial suggests though.
I had seen the [ G-Codes ] button in Mach3 (and used it) - I just hadn't noticed that the list was larger than the one I had downloaded
Looking again at the example you site TPS, it seems to me that it would not drill a hole at the initial position but would drill holes in a diagonal line rather than a grid.
By omitting the Y parameter I would expect 3 holes along the X axis at 4 units pitch and of course omitting the X parameter - 3 along the Y axis at 5 units pitch.
Hmmm... I think I need to do a simulation and watch the 'Table Display' very closely.