VTOL Quad Tail sitter - Elevon Faults

I have built an X configuration tail sitter.

It works well in MC mode. I have not completed full free flight tests in FW mode as during testing the left elevon receives a command to centre (PWM 1500 micro secs) the other Elevon seems to stabilise.

I have been following the procedure for bench transition testing as follows:
Select the VTOL mode switch - the craft pitches forward as expected,
I then blow into the pitot tube, and it completes the transition.

The mixer file has a null command (Z:) I am about to try with this removed.

Does this sound right?