Note to anyone using the above technique that "extension hardening" from Office 2007 onwards now throws up a warning every time you try to open the file:
"The file you are trying to open, '...xls', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?"
Selecting yes opens the file as normal...but does anyone know a way around this?? From what I can tell the only fix is a registry hack which works for Office 2007 but not Office 2010.