Charlie,
The problem might be one of timeouts.
Standard broadband lines are asynchronous, meaning that you may have 10 or 20Mb download speed but your upload speed will be about 5% of that!!!!!! (0.25Mb or 0.5Mb)
So there lies a potential problem, you may be hitting the time limit for uploading. Of course if we all lived in the US and had super fast up and down speeds then it would not be an issue!
Try uploading a shorter file (thus smaller) just to see if that works.
Let us know how you get on.
Rob