@dc42 Thank you that fixed it.

I see how that would work, not sure whether to feel embarrassed at not having spotted it - or peeved that it accepted a 'default' that I never intended .

Sorry to have troubled you!

I tried allocating the 'unused' motors - made no difference.
Having looked at the schematic I understand why hardware fault seems "most unlikely"!

I can confirm that a motor connected to P0 rotates in sympathy with Z axis - P7,8,9 - whether it is allocated to an 'E' or not, config.g: (Max speed on X,Y is probably too high - was experimenting)

; Configuration file for Duet Ethernet (firmware version 1.20 or newer)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool on Sun Apr 15 2018 16:25:27 GMT+0100 (GMT Daylight Time)
; Much modified since

; General preferences
G90 ; Send absolute coordinates...
M83 ; ...but relative extruder moves

; Network
M550 PJWB_DBot_1 ; Set machine name
M551 Ppasswordhere ; Set password
M540 PBE:63:4C:40:53:55 ; Set MAC address

;Working off network main presently
;M552 P0.0.0.0 S1 ; Enable network and acquire dynamic address via DHCP
M552 P10.0.0.28 S1

M586 P0 S1 ; Enable HTTP
M586 P1 S0 ; Disable FTP
M586 P2 S0 ; Disable Telnet

; Drives
M569 P0 S0 ; Drive 0 goes forwards
M569 P1 S0 ; Drive 1 goes forwards
M569 P2 S1 ; Drive 2 goes forwards
M569 P3 S1 ; Drive 3 goes forwards
M569 P4 S1 ; Drive 4 goes forwards
M569 P5 S0
M569 P6 S0
M569 P7 S0
M569 P8 S0
M569 P9 S0

M667 S1 ; CoreXY

M584 X6 Y1 Z:7:8:9 E3:4:0:5:2 ; Apply custom drive mapping
M350 X16 Y16 Z16 E16:16 I1 ; Configure microstepping with interpolation
M92 X100 Y100 Z400 E600:600 ; Set steps per mm
M566 X900 Y900 Z12 E36:36 ; Set maximum instantaneous speed changes (mm/min)
M203 X50000 Y50000 Z180 E1500:1500 ; Set maximum speeds (mm/min)
M201 X1000 Y1000 Z250 E120:120 ; Set accelerations (mm/s^2)
M906 X1200 Y1200 Z800:800:800 E800:800 I30 ; Set motor currents (mA) and motor idle factor in per cent

M671 X150:354:-54 Y-71.5:282.5:282.5 S3 ; leadscrews at rear middle, front left and front right; max correction (1 is default)

M84 S30 ; Set idle timeout

; Axis Limits
M208 X0 Y25 Z0 S1 ; Set axis minima
M208 X300 Y300 Z300 S0 ; Set axis maxima

; Endstops
M574 X1 Y2 S0 ; Set endstops (y = max), XY active low

; Z-Probe
M574 Z1 S2 ; Set endstops controlled by probe
M558 P9 H3 F200 T5000 ; Homing Z axis, P9 for BLTouch
G31 P5 X-36 Y20 Z1 ; Set Z probe offset and trigger height,Set the G31 P parameter to a low value, because the BLTouch produces only a short output pulse
M557 X36:264 Y45:280 S20 ; Define mesh grid
M307 H7 A-1 C-1 D-1 ; Disable Duex heater 5 (used for BLTouch, see deploy & retract probe.g)

; Heaters - not yet set up
M301 H0 S1.00 P10 I0.1 D200 T0.4 W180 B30 ; Use PID on bed heater (may require further tuning)
M305 P0 T100000 B4725 C7.060000e-8 R4700 ; Set thermistor + ADC parameters for heater 0
M143 H0 S120 ; Set temperature limit for heater 0 to 120C
M305 P1 T100000 B4725 C7.060000e-8 R4700 ; Set thermistor + ADC parameters for heater 1
M143 H1 S280 ; Set temperature limit for heater 1 to 280C
M305 P2 T100000 B4725 C7.060000e-8 R4700 ; Set thermistor + ADC parameters for heater 2
M143 H2 S280 ; Set temperature limit for heater 2 to 280C

; Fans - not yet configured
M106 P0 S0.3 I0 F500 H-1 ; Set fan 0 value, PWM signal inversion and frequency. Thermostatic control is turned off
M106 P1 S1 I0 F500 H1:2 T45 ; Set fan 1 value, PWM signal inversion and frequency. Thermostatic control is turned on
M106 P2 S1 I0 F500 H1:2 T45 ; Set fan 2 value, PWM signal inversion and frequency. Thermostatic control is turned on

; Tools - not yet configured
M563 P0 D0 H1 ; Define tool 0
G10 P0 X0 Y0 Z0 ; Set tool 0 axis offsets
G10 P0 R0 S0 ; Set initial tool 0 active and standby temperatures to 0C
M563 P1 D1 H2 ; Define tool 1
G10 P1 X0 Y0 Z0 ; Set tool 1 axis offsets
G10 P1 R0 S0 ; Set initial tool 1 active and standby temperatures to 0C

; Automatic saving after power loss is not enabled

; Custom settings are not configured

; Miscellaneous
M501 ; Load saved parameters from non-volatile memory

Duet3D Eth + Duex5

Motor connected to P0 moves when P 7,8,9 are stepping.
Only this driver is affected - there is a good earth between the boards.

CoreXY - P7,8,9 are used for 3 lead screws Z axis - P1 is 'Y' - currently P6 is 'X' .

I can confirm that this unwanted movement happens even if P0 is not "in use" - I connected an extruder motor to P0, whilst 'X' was on P6 - it moved when the Z axis moves.

This is a new board, nothing has been plugged/unplugged with power on. I have made no further attempts to diagnose (no scoping!).

Any suggestions would be welcomed - thanks