FTP is probably going to be your best option. The only caveat there is that you'll want to:
1) Make sure the workbook name is static, so no dates or anything appended to it. This will allow it to have the same name each week.
2) Whenever the next report is expected to arrive, that the existing workbook is removed before then. This could either take place with you logging in to the FTP site and renaming/deleting it, or working with your IT team to have a cron job run that automatically deletes it each week.