This was an issue with the max post settings on that server, nothing in the script. They were set to:
post_max_size 8M
upload_max_filesize 6GB
Something in the 6GB value was causing the issue as when I reverted it to 500M (using the local php.ini file) the plugin upload worked fine. It was probably failing to read it which reverted it to zero. I'd recommend using it in the format 6G or 6144M.
Consider also the 2GB restriction on 32bit file systems, so you may only be able to set 2G at most.