The file I have is not supposed to be a shared file. I expect when I have
the file open, someone else trying to open it will receive the standard
message saying it's already open, and do they want to open it as read only or
be notified when it's free etc.

Sometimes this happens.

Sometimes, however, it happily lets my workmate open the spreadsheet and
make changes, causing all of my changes made since my last save unsaveable.

Any ideas on why this might be happening. My IT department gave me some
very useful notes for Excel 2003 which are not relevant in 2007, and I can't
find anywhere where I might turn sharing off anyway.

Thanks

Maria :)