recover damaged keynote file

Several times this has happened to me: Saving a keynote 09 file; gets near to end of saving process, error message saying "an error has occurred, quit Keynote". File doesn't save and destroys the original version – when the file is opened in LKeynote after restart, a message is displayed saying "unable to open not a valid keynote file".

Have seen previous posts wehere people suggest changing extension to .zip, etc, but has anyone worked out this problem?

I have just lost a days' work on the final save. Angry doesn't begin to cover it.