Definitely hear the sound of the autopilot disconnecting so we can discount the idea of an auto land gone wrong due to localiser interference. The visibility at the beginning doesn't look that bad but appears to deteriorate fairly rapidly as they get closer in. I would say they lost visual reference in the last few seconds and drifted off the center line without realising it until they were in the grass. The runway is 45m wide so a drift of a little over 20m will have you departing the pavement. This wouldn't be too difficult if both heads were looking out trying to acquire the runway and no one was monitoring the instruments. Slant range effect may also have paid a part.