Wobbly lines on static camera videos
After much help above it looks like its because the camera I was recording on was recording in 50i but the project I had created in fcpx was set to 60p, this was causing issues with the final result as it tried to convert 50i to 60p...

I restarted one of the projects as 25p instead and copied in some of the source video again and a quick repeat of my normal output process (export as mov, use handbrake to convert to a reduced size mp4 for youtube upload) seems to give a much better result (I'm comparing the mp4 on my machine to youtube but there is 0 wobble and I don't think its youtube adding it in) so I think this is the solution, make sure your projects are set to the same fps as the source!

Thanks all