Thanks for that advice, I was starting to think that was the case. I did try and Map to some other OEM LEDs to check my process was correct and was unable to get any of them I tried to work with the method I described above. Is that the correct process?

Yes, your approach is correct. Unfortunately, it is unknown to me which OEM LEDs allow you to do this and which ones don't.

Just tried with a momentary switch and it is working fine here.
Alex

Hi Alex, Yes Momentary switches work fine, and I am using them for lots of other functions.
What I wanted to do was to use a toggle switch to control these functions so that pos "off" forced block delete off and position "on" forced it on. I have been able to design a little circuit that emulates a momentary switch , sends a low pulse on each change of state of the switch (on - off or off - on) and this works great, just like a momentary, except it doesn't force the state (off position on the switch can be in fact on)
I'm now looking at creating separate on/off buttons for these in mach3 and mapping 2 inputs from the pokeys to them. That's todays project, so I will report back on that one.

I understand now  what you are trying, but as far as I know this will not work because the Block Delete function is a toggeling function within Mach3.
Or in other words it can't be forced to stay on or off.
Alex

Correct, the current button in the standard set has a toggling effect.
I have created 2 new buttons in Mach3 using Screen4 with some VB script and one turns it on, one turns it off. This works well. Now I just need to map a pokeys input to these new buttons, and that is where I am struggling at the moment.
I can get it to work with assigning hot keys, but there is another issue in that these are ignored if Mach3 does not have the focus on windows.
I am starting to think illuminated press buttons will have to be the answer, but finding some with enough colour range at the right price is a challenge in itself.