@rpdom - Hello Essex! I am just over the northern border
I believe the kernel panic is "expected behaviour" as the poweroff script stops the Pi from completely shutting down and I believe there is a timer in the kernel which fires a panic as things haven't turned off in time.
I don't have a console connection on mine at the moment, but I do remember looking into this with Jessie earlier in the year.
FYI - I used the default pin - GPIO26 (Physical 37)