Well a bit of progress playing about with MACH3 screens now I've found a bit of time
I've managed to create custom buttons to swap from page to page, also managed to tie them into keystrokes to avoid using the mouse, learnt that buttons on Screen 0 are 'persistant' so appear on all pages, and also I've managed to pinch the 'diagnostics page' from standard page layout and add it to my screen set - all fairly simple stuff, but it's giving me a 'peep under the curtain' to whet my appetite.
However, I've also learnt that whoever created the Purelogic screenset that I'm using with my CNC Plasma Table put an awful lot of the DRO's and other controls on Page 0, so (for instance) my diagnostics page is over written with this information
At this stage I'm playing with an 'offline copy' of my screensets that I've put on a second PC to avoid messing anything up seriously, as I can easily re-copy the original files, but this also means that Mach3 isn't connected to anything to test my diagnostic screen.
Still, I'm quite pleased and extend a thank you to all those who pointed me in the right direction.