Sorry for the delay - new job means this has taken a back seat fro a while.
I took a good look last night, and still have no idea ! I disabled the macropump and loaded the unmodified 1024 screenset, the reaction was still the same - any spindle S words were accepted, the DRO was updated, but the physical spindle speed did not change until a spindle speed override button was pressed.

i gave up !

The xml and other bits/pieces were copied out, and a complete uninstall/reinstall of mach completed, using the same version (3.042.025). It works now !

So, as I said - I'm no further forward in fault diagnosis, but the symptoms are fixed, using the same macropump, same gcode, same bitmaps and same screenset. There must have been a setting in there somewhere that i'd accidentaly changed.

Anyway - thanks for all the people that thought about this, and offered suggestions.
Tim