BlindBob, there is no doubt that a helicopter downwash can affect light fixed wing and the heavier the helo the worse it gets. All military helo pilots are constantly reminded of the dangers of downwash because you can score some spectacular own goals with your own troops if you are not careful.
However, in this case, the downwash of the Sea King was shown to have remained completely clear of the Cessna at all times and WAS NOT A FACTOR in the crash. The AAIB investigation was quite clear on the cause of the accident.