MightyGem,
At Heathrow we've successfully argued that the 760m value does not apply to helicopter operations. We operate a helipad 250m from our southern runway independent from any vortex (apart from Super Heavy category a/c).
We used ICAO documents (can't remember off hand which one!) where that 250m figure came from (from the edge of a Code E Runway). Our runways are Code F, so hence why we still apply vortex from A380s.
PM me for more info if you wish.