Right -
I rewired all three steppers - to two single coils, refitted them all.
Great - I can now get 16 in per minute on all axis. This is fabulous - 4 times what I was getting on bipolar series wiring.
This for me is fine. My mill table is only 12" by an effective 10" so I can move any where in 45 secs. In lathe mode the length of the leadscrew is 18" so I can move from end to end in about a minute.
Thanks for the other suggestions, but altering the gearing, which are timing belts is a bit difficult, because I have glued the gear onto the motor shaft, although this would increase the speed. The Z axis I would leave because I want the torque when drilling and it can be up and down in seconds.
The voltage increase is also a bit difficult, because I use a pair of traction batteries to provide a very steady 24 volts.
That leaves the leadscrew, which I will replace eventually, to cut backlash, and I will certainly consider increasing the pitch of that.
If there are any more suggestions thanks - and thanks for the help etc already - a fourfold increase for no expenditure is excellent.
Jim.