If you departed in a C152 behind one or more Melins, I would have to use 2 minutes wake turb spacing assuming same departure position. (actually, we class all light/medium helos as "medium" in relation to light aircraft)
Don't know if the RAF use WT seperation.
Rotorwash do travel down and out, and affect other aircraft quite simmilar to wingtip wake. It does drift downwind, but I belive at a slower rate then normal wake.