Good news and Bad news............
Good, I have the mill under control from mach3 via the UC400ETH
I had to fiddle around a bit, the CNCDrive manual didn't work perfectly for me.
Bad, I've discovered I hadn't kept a backup of my mach3 machine profile somewhere other than on the computer that I used to control it (and which died a sudden death)
So I am plodding through ALL the settings, including faffing with the acceleration etc.
I'm sure everyone else is much more organised than me, but if you haven't got your profile saved somewhere else, I really recommend you do it asap!