We would like to have a workbook stored in our ISPs
server (accessable using FTP & SFTP). Is there a
way to access the workbook so that either locking
or shared workbook editing would work?

I experimented with opening the file directly over FTP
from Excel file dialog and sharing the folder over SFTP
using SFTPDrive. However neither one of those methods
supported file locking thus if several people edit the
workbook simultaneously some changes will get lost.

Do you have any ideas how to solve this issue?