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I believe I've tried almost everything... still getting the same error from two different modules, I only have two modules...
I've searched and searched and tried combinations for days. Any suggestions welcome!

This is the error in the log of the mcu flasher (latest version):

Note:Serial port connected.
Note:Begin find ESP8266.
Note:ESP8266 ACK success.
Error:Read ESP8266 register timeout.

CH340 (possibly G) in 3.3 ttl mode (via a jumper)
External 1amp 5v power supply, DC to DC buck converted to 3.3v with 470uF electrolytic cap on output.
All negatives are at a shared common ground (USB, buck, PS and ESP)

The usual connections:
USB -> ESP
TX - RX
RX - TX
VC C- VCC
VCC - CH_PD
GND - GND
GND - GPIO0

100nF ceramic cap between VCC and GND close to ESP board as possible.

When GPIO0 is left floating, all works well, I can AT it and connect to a wifi network etc.

When GPIO0 is at GND, I can't communicate with it within the terminal window, confirming some kind of change.

I've tried with and without the capacitors.
I've tried at a slightly lower voltage (3v)
I've tried with and without a 10k resistor for the CH_PD - VCC link
I've confirmed I didn't leave any software hanging on to the port before launching the flash util.

I've followed tutorial after tutorial.

Purchased last week at aliexpress
This is the AT+GMR

AT version:0.40.0.0(Aug 8 2015 14:45:58)
SDK version:1.3.0
Ai-Thinker Technology Co.,Ltd.
Build:1.3.0.2 Sep 11 2015 11:48:04

Not sure what else to say :-/

I changed the ttl device to a CP2102 chip based board, and flashing now works great.
The CP2102 appears to be 3.3v TTL signal - fantastic.

The offending CH340 chip based board is called a 'D-SUN-V3.0 USB TO TTL' - and although works with the AT COMMANDS at 115k and 9600, will not flash....

Hope this helps someone