Gubar: The day before yesterday, I played with a camera and simple cube to see the difference between 'parenting', and a 'parent constraint.' It is the 'weight' attribute isnt it, that you would set a key on for this method? Its a floating point value between 0 and 1? So I would have the weighting of the constraint set to 1 right until the lift stopped, and then the following frame would key right back at 0? I think at the time I decided not to use the parent constraint, because I was worried that once the weighting was keyed off, the helicopter would tumble back down to the floor again....???
Tweet: I ended up doing just what you suggested. Duplicated the scene file, so that I can animate the lift rise part seperate from the take-off part. Same lights, render settings, camera etc. Broke the connections on the helicopters translate channels, and then attached to the motion path. Which actually worked slightly better than I thought it would, although hand keying the rotation of the helicopter as it travels the path was difficult and left me with a very messy looking timeline. The 'bank' attribute didnt seem to do alot. And the helicopter seems to speed along the first third of the curve and then take it's own sweet time travelling the rest of it. Whether this is just perspective, as it travels away to the off right, or actual real speed, Im not quite sure. i think I read there is a way you can control the spacing of the curve to alter the speed, but not quite got that far yet.
All: Cheers for the comments, I'll upload the playblast I have so far and then post a link here, if I cant adjust the timing of the motion along the path, I'll break it all and try again, or another method.