ok, so the instructions say "Turn on the transmitter, then connect the power of the receiver keeping the receiver "bind" button till the light turn on GREEN which means the binding is successful."
however thank you for suggesting trying both powering on at the same time whilst holding the bind button...
It worked, so the receiver is good, the instructions are broken
Interestingly I could bind one receiver by following the instructions, however the second one needed to be powered on at the same time in order to get it to bind....