Next brainwave, see if the camera can add silence and just copy the stream, same as the video. Well, I remember there was a setting and eventually I found it:
I ticked this, saved, changed the command line to simply copy the stream and voila! Down to 3% and streaming perfectly.
As the Pi isn't quite ready for production use at home I went back to my original server, updated libav-tools and the dependencies and made the same changes and I was greatly relieved to see the same behaviour. All is good, finally.
The underlying command line is now like this:
avconv -v warning -rtsp_transport tcp -i rtsp://camera1:8554/CH001.sdp -codec:v copy -codec:a copy -f flv rtmp://a.rtmp.youtube.com/live2/publish-id -c copy -map 0:v -f segment -segment_time 300 camera1-%03d.mp4Note that the last part of the command line is to do with the saving off or 5 minute chunks as a CCTV backup and everything after the -c copy ... can be left out