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

I'm very new to this, had some problems with Uno and an Ethernet shield (it's getting replaced) so decided to try out an ESP8266 as an MQTT Client Gateway. Followed the instructions with mySensors2.0.0, builds find, but I can't upload the sketch. This is the error I get:

WARNING: Category '' in library UIPEthernet is not valid. Setting to 'Uncategorized'

Sketch uses 244,441 bytes (23%) of program storage space. Maximum is 1,044,464 bytes.
Global variables use 34,036 bytes (41%) of dynamic memory, leaving 47,884 bytes for local variables. Maximum is 81,920 bytes.
Uploading 248592 bytes from /var/folders/fj/hzfwt74921v57hfdlw926rcw0000gn/T/build2f54fb14bd01c3e2d9f3141ffbe7dce4.tmp/GatewayESP8266MQTTClient.ino.bin to flash at 0x00000000
warning: espcomm_send_command: didn't receive command response
warning: espcomm_send_command(FLASH_DOWNLOAD_DATA) failed
warning: espcomm_send_command: didn't receive command response
error: espcomm_upload_mem failed

I'm uploading from a MacbookPro over USB with upload speed 9600.
This is the board I got, An ESP8266 12E Lua.

Any ideas how to debug this?
Thanks
Christian