Put1992
In BA Longhaul at LHR (where I am a CSD), the sequence of events is ;-
Once the cabin & galleys have been checked as all secure for take off, I will go to my crew seat, inform the flt. crew that we are secure for take off, & make a PA to tell my crew to take their seats for departure. After this, we should also get the double chime from our flt crew to inform the c.crew that we are turning on to the runway & that take off is imminent. By this time, all c.crew should be strapped in.
I believe that the "double ding thing" is a "nice to have" & not a "must have" hence why the CSD tells the crew to take their seats, & not necessarily wait for the double ding.
It may well be different for other airlines & even s/haul on BA.
Matt.