I think I may have solved it. In the export dialogue for MPEG Streamclip, there’s a box that says Interlaced Scaling, and above it it says ‘Deselect for progressive movies. Did a test and with said boxed unticked and it’s working. The odd thing though is for my rushes, all the footage from the first two days were giving me the render issue whereas the last two days were fine (just noticed that after your last response). I had batched transcoded them all together so don’t know why there was a discrepancy.
Regardless, unticking the box for the first two days rushes looks like it works so I can move forward.
Thanks for your time,
D’Arcy