I've disabled console on the GPIO serial interface, lowered the speed to 9600 and had some communications observed with minicom and a simple
talking to an arduino all ok.
Now for some reason the Pi crashed and upon hard reboot if I try to use /dev/ttyAMA0 in any way, eg configure the speed with
stty -F /dev/ttyAMA0 9600
the Pi crashes immediately and requires the power reconnected to get it running again.
Voltage level conversion between rPi 3.3v & Arduino 5v was carried out with a MOSFET 2N7000 based design explained here: http://www.fritz-hut.com/connecting-an-arduino-and-raspberry-pi/
Any suggestions as I'm stumped with this one.