Joe,
I've spent many hours hovering by day and by night over various places, some of them not too far from where this "Airprox" occurred, some of it doing the same job.
Helicopter rotorwash doesn't go laterally round and round the aircraft like you appear to think it might. Downwash from a hovering helicopter in free air goes down and away. When hovering near the ground, where recirculation can occur, something light such as a paper bag might go down through the rotors, get blown up again and through a second or even a third time. However, in thirty years of flying helicopters and being around them on the ground, I've never seen something go round and round the aircraft, clockwise or anticlockwise.
I think it's most likely to have been someone messing about with a R/C model, despite what the modelling experts said in the report. Not too long ago I reported a R/C model aeroplane inside the London Heathrow control zone as it went past me at 1500 feet. I understand that R/C models should not officially be flown above 400 feet above ground but models are easily capable of flying much higher; the model flyer has no means of gauging height.