Thank you SO much for your help. I've actually come up with a reason/solution which might be worth adding into any support material - the bits where it says about troubleshooting and file types? This is obviously me, a non techy talking, but maybe it can be translated into proper language!
So I found out there are different types of MP4s? HD, VGA, SD, HQVGA and one which I've got here as 'low' whatever any of that means. If you save your MP4 as 'low' it works in ES. I presume this relates to file sizes or some such and the rest are too big for ES? I probably sound really thick putting