I think I've got there Ian - the mpg is rendering at the moment. I did a series of experimental clips with different settings. I think that interlacing was certainly part (if not all) of the problem. The video is 4:3 aspect ratio, but somehow I had got the project setting as 16:9 in the video editor. Having changed that back to 4:3 I managed to produce relatively video. I also had the interlacing setting to "interlaced, top field first" which I think may have been wrong because when I use the detect interlacing setting in the video editor, it sets that value to "interlaced, bottom field first". To be honest, I tried both settings in my experiments and couldn't see much difference. I then had to get rid of the nasty line near the bottom, so looked on videohelp and found a suggestion there which was to make a full frame mask with a transparent centre and black bars top and bottom to just hide the video interference. I did that by making a .png file in photoshop and then adding that to the effects track in the video editor. This looks OK and hopefully the final DVD will be fine.
I really appreciate your help here...I was tearing my hair out...
regards,
Tony