$ raspistill -vf -hf -ISO 200 --nopreview -ss 6000000 -br 80 -co 100 -o out6s200ISO.jpeg
Camera control callback cmd=0x4f525245mmal: No data received from sensor. Check all connections, including the Sunny one on the camera board
Reconnecting battery and all connections seemed to resolve this. So was either a loose connection, or somehow the setup had got confused, and wasn't resetting as it continuously had power from the battery.