Thanks Jeff,
I’m using four different types of clips in my project, two archival footage and two different interviews shot by two different people.
The first archival footage clip is mpg 25fps 720×576
The second archival footage clip is .mov 25fps 720×576
The first interview footage clip is .mov 25fps 1024×576
The second interview footage clip is .wmv 25fps 1024×576
The editing sequence settings are DV Pal, 25fps, 720×576
Export settings are H.264, pal, 1280×720, 25fps, upper field first
Here is a screen shot of the sequence settings

Here is a screen shot of the export

I hope this is helpful, I’m thinking it must be something to do with the order of the fields but I can’t figure it out having tested multiple export settings.
Cheers