Recover deleted .AVI file from a user's desktop (with FileVault Protection)

Okay, here's the situation....

- Downloaded a video file from Camera flash card directly onto the desktop of a user account set up with FileVault Protection

- File was deleted from desktop and Trash emptied (but not using secure empty)

- Tried using Data Recovery 3 and TechTool Pro 5 to recover data from Mac HD but to no avail

The closest I've got to achieving a recovery is via the beta version of Disk Drill as I was able to use that to try a data recovery from the Camera's flash card. However, after recovering the file onto an external HD and trying to play it, I get a message that it's not a movie file. This is despite the fact that it still has the .AVI extension and icon with the correct filename. However, the recovered file is 1.09GB in size, so I figure there must be some video data in there somewhere?!?!

Have just downloaded Boomerang Data Recovery and it also sees the file on the Camera's flash card, but I can't recover it without purchasing an activation code and I'm not prepared to spend the hefty $99 without knowing if it's going to work.

Any suggestions would be most welcome and appreciated!!!

Thanks.

Joe.