Hi again, many thanks to i2i for the chat and the info, very useful.

However after a great deal of thinking I've decided to stick with the CDE system, mainly for the following reasons:

1/ I want to do rigid tapping of threads down to M3 and the CDE system has a 'many pulses per rev' input and will adjust the feed to match the spindle even if the spindle speed changes, as it inevitably will when the tap picks up the thread. A single pulse per rev is not as good although it would be fine with a tapping head.
2/ I can still retain most of the 190VMC control system using CDEs XC641SC board to generate the correct pulses for the SGS drivers.
3/ The CDE spindle control is built-in with analogue output.
4/ The CDE system has inputs for limit switches in all directions.
5/ I can retain the front panel jog switches
6/ I can retain the front panel feed rate control knob, which I have found very useful on the one CNC machine I get to play with occasionally. It's very handy to be able to slow things down just by reaching for a control knob, although I am sure Mach3 has a similar function it might not be as easy (quick) to operate.

Thanks again to i2i
David