Rich,
The codec won’t really have any effect on this apart from some blocking maybe if you were in DV, which you are not. I work in SD 16:9 PAL all the time and see this all the time when there is a lot of movement.
Have you got your canvas set to 100% if so make it 99% and see if the effect goes away.
Having said all that, it won’t hurt to check your settings again and/or apply the easy setup
Peter