As an addendum, the issues I was having with my z axis during homing were due to the endstop. As the endstop I have was designed to work with 5v (I changed the 1k resistor down to 330) the IR LED wasn't quite as bright running off 3.3v. there was some plastic flashing on the plastic cover that was obstructing the IR receiver, cleaning up the flashing with a hobby knife has resolved the issues and I'm able to home with the new board with properly working drivers.
Thanks again for all your help DC42