Thanks for the quick reply overeasy 
I did sort this out and sharing my findings incase anyone else comes across it. The ESP8266 uses pin 15 (D8), which the IoTaWatt uses for the SD Card CS pin as a ‘boot from SD card’ hardware option if it’s high for 250ms during boot. The -01 series incorporate a 10k pull down resistor between pin 15 and GND on the board however the -12 series do not. This is characterised by the device not booting and the ESP console monitor hanging at “waiting for host” when an SD card is installed.
This is easily overcome by putting a 10k resistor between D8 and GND.