I posted the original question. Thanks for all the answers. I was asking as I was operating out of LHR and departed immediately, in a heavy, behind another heavy. When I say immediate I mean we cleared to take off as the preceding aircraft got airborne! That's not a criticism by the way as I appreciate the excellent service we get from London controllers.
Anyway, we had a mild wake turbulence encounter just as we got airborne. Nothing dramatic but we filed an ASR as per our SOP and that got me looking in our Ops Manual and had me wondering what exactly the rules are regarding separation for wake turbulence as opposed to traffic separation. I suspect the company manual has lifted the rules directly form the relevant ICAO/CAA document for these matters. I suppose the lesson is beware of wake turbulence at all times.
regards
BBK