Stupidity on my part, realised that gst_loopback was set to /dev/video1 by default, changing that to /dev/video0 meant that FFmpeg then works if you set the input device to /dev/video0. Streaming to YouTube working. If anyone is interested this is the FFmpeg command
ffmpeg -f lavfi -i anullsrc
-f v4l2 -s 3480x1920 -r 10 -i /dev/video0
-vcodec libx264 -pix_fmt yuv420p -preset ultrafast
-strict experimental -r 25 -g 20 -b:v 2500k
-codec:a libmp3lame -ar 44100 -b:a 11025 -bufsize 512k
-f flv rtmp://a.rtmp.youtube.com/live2/secret-key
Will need some work on optimisation but gives me what I need.