Dual X carraige second layer error on Z axis

I have a home built P3Steel which I recently modified to a dual X carraige.
I have happily been printing with the primary extruder for some time with no issues.

I needed to use both extruders today for a part with support material of a different type but after tuning M218 to correct alignment issues between the two extruders, It would print the first layer well but then try to print the second layer about 1.5mm above the first.

I have M851 set to account for the height difference between the proximity sensor and the T0 nozzle, and use M218 to account for the height difference between T0 and T1 nozzles. It seems M218 T1 Z? is the height difference between the two nozzles plus the probe offset).

I tried setting M851 to 0 and setting M206 instead but that did not work.

I am running Bugfix-1.1.x

Output of M503:

G21    ; (mm)
M149 C ; Units in Celsius
Filament settings: Disabled
M200 D1.75
M200 T1 D1.75
M200 D0
Steps per unit:
M92 X80.00 Y80.00 Z4000.00 E94.00
Maximum feedrates (units/s):
M203 X300.00 Y300.00 Z2.00 E25.00
Maximum Acceleration (units/s2):
M201 X3000 Y3000 Z100 E10000
Acceleration (units/s2): P R T
M204 P3000.00 R3000.00 T3000.00
Advanced: B S T X Y Z E
M205 B20000 S0.00 T0.00 X10.00 Y10.00 Z0.30 E5.00
Home offset:
M206 X0.00 Y0.00 Z0.00
Hotend offsets:
M218 T1 X-0.40 Y0.00 Z0.95
Auto Bed Leveling:
M420 S0 Z0.00
G29 W I0 J0 Z-0.35125
G29 W I1 J0 Z-0.26350
G29 W I2 J0 Z-0.14125
G29 W I0 J1 Z-0.02000
G29 W I1 J1 Z0.01250
G29 W I2 J1 Z-0.02000
G29 W I0 J2 Z0.14825
G29 W I1 J2 Z0.08300
G29 W I2 J2 Z0.02175
Material heatup parameters:
M145 S0 H180 B70 F0
M145 S1 H240 B110 F0
PID settings:
M301 P22.20 I1.08 D114.00
Z-Probe Offset (mm):
M851 Z-0.85

Any ideas why it would change height and print the second layer in mid air would be appreciated.

Thanks in advance.