Well its a bit hit and miss but I finally managed to get my echo to connect as sink.
I rebooted Echo, clicked remove on Bluetooth Max2Play tab.
In a terminal window I ran bluetoothctl then scan on, then prepared the command to connect ready to hit return. Then quickly said Alexa Pair and immediately hit return to issue connect command in terminal.
To my surprise, this time it connected as a2dp sink. And of course I can now play music.
Has survived disconnect/connect and pi reboot.
It’s like if you give Echo too long to think it will assert itself as source and pi lets it.