The errors continued despite the flash size settings change… yet running ESPTOOL.py erase_flash and reflashing firmware and SPIFFS helped for a bit… errors would return after a couple of power cycles… however it seems that what’s resolved this has been updating ArduinoIDE to 1.8.1 (I was on 1.6.12) and reinstalling the FTDI drivers.
When power-cycling to upload SPIFFS, it was to necessary to unplug the USB to power cycle the FTDI adaptor as well as the ESP chip.
Sorted! Both prototype boards working 