... I've now worked out that deposit files are expecting to download to my ISP's IP address, not to mine. Deposit Files have also not responded to the "ticket" I raised with them.  

The paranoid in me is saying that my ISP is monitoring what I download (because my traffic is heavy); I was put on a slower download speed because of heavy use last month. I will keep me head down for a while, and see what transpires - I'm not very confident about complaining to my ISP about not being able to download for free what is probably copyright material (which has entered the public domain by dubious means)!