HI, Thanks for getting back to me.
Of note, the my sensors v.15 is actually 2.0.0 from the downloads.
I have a 10k resistor from D0 to Gnd as suggested on various newsgroups, managed after about 5 attempts to flash the ESP8266 blink sketch, then it managed to flash the MQTT Client Gateway sketch after that.
I'm not sure which of those things fixed it, but it's working now. Or it's uploaded anyway.
Thanks
Christian