Hi Jeff,
Thanks for your reply! The footage recorded in a .MTS format, from which I imported from camera into FCPX and put a back up on my computer just in case.
The actual preview of the footage in FCPX is perfect, and not interlaced at all. I also made sure during import into the timeline that it was set to change the settings based on the clips.
Also – what I find is strange is when I use the ‘share’ option to YouTube, the footage is fine and non-interlaced; but as soon as I export to master file to the both the Source (ProRes 422) and also H.264 the footage when previewed in both Quicktime Player and AVC is interlaced. Also, when that Master File is uploaded to YouTube, it is interlaced.
I’m now making two videos for online use, and still – the interlace issue is here. What makes it worse is FCPX is not allowing me to upload directly to YouTube!
Here’s a screenshot of the Master File, which interlaces during any movement: 
What makes it even worse is I don’t have the raw footage from the camera, as the camera isn’t mine! When I go into the project folder in FCPX where the footage is saved, that too is interlaced?
Is there any way to make the footage deinterlaced once it has been exported, or to fix it in the edit so that once it is exported it isn’t interlaced?
Also, apologies if it’s very obvious – I am new to this!