Ferret - I can also vouch for the strength of downdraft, having had to move from well outside the downwash to directly underneath for ext PA testing, a/c in 60 ft hover. It gets very difficult to walk forwards, almost to the point of being impossible. But, much like a hurricane, it gets suddenly easier near the underneath. I have also watched others try to do this (from the comfort of looking through the RAF variant floor), so it wasn't just me being lardy and unfit.
Did they manage to get you in without banging into the sponson, or using the wheel as a handy support?
I know wind speed measurements in the downwash were done, but haven't got a clue what the results were.