Thanks @WickedShell, I have loaded the plane FW and will be testing in the next couple of days.
@WickedShell did some ground tests and the FC crashed in a few minutes.
Seems to be pointing to some hardware issue.
I did reload Copter 3.4.2-rc2 and it crashed again.
I can find nothing in the logs that gives me any hint as to the cause.
Fitting a new Pixhawk today.
Will continue testing.