Cannot Flash Sonoff ZBBridge with Tasmota - failed to connect to esp8266: timed out waiting for packet header

Hi,
I’m fed up and at my wits end with this but to summarise these are my actions and findings any help or advice that I haven’t already tried would be greatly appreciated.

  • Tried 4 different USB to TTL/Serial adapters - all give the same results
  • All soldered jumpers/pins confirmed as having continuity and/or correct voltage (3.3V)
  • Have tried RX to TX and TX to RX and vice versa, all combinations
  • Have tried plugging the USB in and then grounding IO0
  • Have tried plugging the USB, grounding IO0 and then touching rst to VCC to reboot the Zigbee chip - no difference
  • When attempting to flash with Tasmotizer, the USB TTL Adapter TX led flashes away but nothing received
  • The green LED is lit and solid when attempting to flash, no blue lights

Before attempting to flash the ZBBridge, I have used it unflashed and upgraded the firmware to 1.7.0 - could this be blocking flashing attempts?

Please help!

Surely there must be someone with experience of this?