By way of closing the loop, so to speak, I have had a great response from the CAA following an email question I sent.
There is little empirical evidence that FW wake turbulence affects helicopters in any major way and one study, done by the well-renowned Gareth Padfield in the late 90s, seems to be the basis for applying FW separation criteria to RW.
The problem is that it is theoretical, using vortex simulation algorithms and a Lynx Mk 3 s handling characteristics - a good place to start but it assumes a lot of 'worst-case' scenarios (no vortex dissipation and no pilot intervention for several seconds for instance) and uses a Helicopter handling qualities standard (ADS33) to predict the disturbances in pitch, roll and yaw caused by interaction with the vortices.
Again, good place to start but there seems to have been no attempt to follow it up with real-world testing so anyone reading the study - presented at the 25th European Helicopter Forum - would think that any encounter with any wake vortex would be very hazardous, not something backed up by any empirical data and certainly not confirmed by any anecdotal evidence from real encounters.
Once such a safety protocol is put in place, it becomes near impossible to roll it back or water it down without a huge amount of effort and evidence and the RW industry isn't interested in fighting it's corner here it seems.
It was a useful exercise asking the question and I really wasn't expecting anything to change but I scratched the itch.
Many thanks to Rob at the CAA and to all who have contributed.