Speaking as a controller, my experience would suggest that wake turbulence from helis can be significant (and a significant hazard to light aircraft). AIC 17/1999 (Pink 188) gives a bit of info that might be useful but the diagram would appear to show vortices in flight - I think the effect of heli vortices in low level hover or at lift may be substantially greater.
I remember that there was an accident at Oxford a few years ago. IIRC an aircraft (PA28?) touched down adjacent to a S61 at the holding point and the pilot of the aeroplane was thought to have lost control because of downwash from the rotors.