Thanks Hood,
Yes its UserDRO 1501 to 1503 now.
I have fitted a reset button on screen now and yes it does work as i thought earlier, so all i need to do now is fit a pulse of some sort to trigger the counter each time Mach comes out of reset, that shouldn't be too demanding i think, just a matter of figuring the best way. I am using the 5v supply to the encoder for the pulse to the input of the line driver, still not sure if it should be 0v or 5v though, but it works on 0v.
Back to the main issue...
The new screen-set with user DRO and code moved to a VB button seems to have fixed all the troubles, been messing with it for a good couple of hours and cat get it to lock-up or otherwise stall or mess about. Tool height-setter can be called any time, works every time, macros still work before and after using it, all motion calls still work.
Thanks all