Thank you so much for the quick response. I've been having trouble getting an answer that applies directly to my situation, so I greatly appreciate it.
So I did what you suggested, and here's where I'm at...
There are no options to set the field order in the presets, so I played around with them in the export settings. Upper has the worst results, lower isn't bad and 'using source setting' isn't bad... but they can be quite 'square-ish' around the outlines of a person's body. I'm not sure if I'm describing it well, but it's like the old atari video games - they had a squareness in the graphics - that's what it looks like sometimes, particularly with lots of movement.
I'll give you more info to work with. I chose non-anamorphic for the preset. I chose the quality to be the highest, with the box for 'Use Maximum Render Quality' checked. VBR, 2 Pass, Target Bitrate 7, Max Bitrate 8.
I have left the box for 'Use Frame Blending' unchecked and the Display Format has 'Drop Frame' checked, rather than 'Non Drop Frame'.
I have also left the box beside 'Render at Maximum Depth' unchecked.
Thanks again. I hope this helps!