Thanks. On my original project I used a PNP MOSFET to with a voltage against the gate to keep it off. When the button was pressed it pulled that line to ground switching on the Nano which set a pin to high that went to an NPN MOSFET. This caused it to open, pulling the gate to the PNP MOSFET to ground and ensuring the NANO remained powered. Once the NANO had finished it's task, it set the NPN gate pin to LOW which caused a positive charge to go to the PNP gate and switched the NANO off again. When it did not work, I removed that complexity and just put a button in which passed current to the NANO VIN. This does risk switching off the NANO before it has completed the write. However I leave it pressed down until the acknowledgement LED goes green. I've tried holding it down for many seconds but it still fails.
Wiring diagram. Good call. I'll put something together. Thanks for your help