Note I am deliberately answering my own question
There are lots of detailed guides out there; this is an overview of the key points particularly with respect to using a Raspberry Pi as the host.
Take apart the case and solder wires to the TX, RX and boot_sel2 soldering pads, and make an earth connection somewhere.
Ensure the Raspberry Pi is not trying to ...
Found the answer to my own question.
The Pi UART is by default set to echo mode. Hence the echo behaviour! This presumably reflects its original purpose of working as a tty console. Echo is required for normal console behaviour.
Fix is simply to reconfigure the UART by disabling the echo.
Worked as expected after I'd configured with:
stty -F /dev/...
You directly connected Pins 8 and 10 to the USB-To-TTL-Adapter? That's a bad idea that can fry your Pi. The Pi runs at 3.3V (including its UART-Pins), Uart power level is typically between 5 and 20V. You need to use a voltage translator or a board specifially designed to convert the Raspberry Pi's Uart Pins to TTL level.
This may, in fact, result in the ...