Having imaged the 160 GB HDD, I used HxD to overwrite the first 355 MB with firmware.txt from the thread above without success. I also tried dmr-recover.py script on the image to try and recover the movies without success (probably a different format from the DMR EX77 / US Models).
Panasonic DMR EX773 was never sold in North America: USA/Canada NTSC versions stopped with the EH55 series of 2006. The firmware of later deluxe PAL models like yours was more complex and may require advice from a European, Australian, NZ forum more frequented by owners of the unit. I'm sure someone will chime in here to offer at least partial assistance, as we have a couple of very skilled Panasonic repair techs posting, but you may need to dig a little further (or bring it to Panasonic repair depot).