I have found a couple of versions, one uses g28.1 the other uses g31. The g31 version seems besta st it uses the switch connected to the mach probe input and apparently it can still allow "home all" to use the upper limit switch on the Z axis, this is exactly what I want.

I will start on building a floating head as it seems pretty essential and makes a lot of sense to me.

I also need to find out about editing the sheetcam post file to insert the new code.

am I right in believing I need to use a THC post in sheetcam now instead of the basic mach3 post?