It appears I have solved the problem with corrupted uploads! I finally got through to my ISP, and grilled the tech-support guy for about an hour. He went through all the usual rubbish about how it was probably a fault on my system etc.... I told him I wanted to bypass the transparent proxy server, and he told me it wouldn't be anything to do with that. I INSISTED that he let me bypass it, and when he gave in and let me bypass it I did a test upload, and it appears to be perfect! WOOOOOOOO HOOOOOOOOO
I am going to do a few more uploads and see how it goes.
Once again, thank you to everyone who gave me help and advice with this, I would never have got this far into solving the problem without you.