A quick scan though the Mach2 Custom .PDF seems to indicate that there is a probability that it will contain useful information when I have done a bit more study.
Currently I'm looking carefully at the Mach4 GCode Manual which is certainly more comprehensive than any previous G-Code tutorial I've seen -- specifically detailing the parameters expected for each code and providing explanations as to what each is expected to do.
It may well be that some of the codes have seen embellishment or at least 'tweaking' but I suspect that in general I can use the information for testing.
Having hand-coded a program to cut a circular pocket, clearing all the internal waste material away - and realizing how tedious changing many lines of code can be, not to mention the fact that I now have to do the same to mill the thread, with each pass needing to feed the cutter a little deeper (increased radius) --- I'm considering writing a Program (in Pascal) to enter the various potential variables -- Pocket dia. & depth, Tool dia., Feed rate etc. which will then output the G-Code with all the many lines of code taking account of each 'pass' needed. It won't be a 10 minute exercise though