Ok so I ran G29 another 5 times with the bed and hotend off and still got an incrementing error as before.
Changed the probing speed to F100 as suggested and this has solved the problem, have also changed to P25 from P10 in the G31 line.
Thanks for your help.
New lines:
M558 P9 H3 F100 T6000 ; Set Z Probe to type Switch or Digital output where Z probe connector is used.
G31 X20 Y0 Z1.35 P25 ; Set Z probe trigger value, offset and trigger height