Being honest Daz I dont really understand the ins and outs.
I was advised by a chap I met at the Alexander palace model show( member of the society of experimental engineers and I understand he was their CNC guru) that in order to run Mach 3 on a 64 bit machine I needed a UC100 . It would not drive even a single stepper when I first tried .
I took his advice and it then worked.
Same gentleman guided me here so his advice seems sound enough.
He may well be reading this thread !