Thanks Jay; the flock is mostly integrated now, after a rocky start. I wanted the newbies to be familiar with the whole garden and their new life, so initially I was keeping the mated pair locked up in their yard while they free ranged all day; then everyone locked up in two separate yards overnight. After about a week of this, the drake began breaking out of his yard in the middle of the night and into the newbies' (my small daughter was convinced he flew in and out). After several nights of this, we just began locking them all up together, in the newbies' yard. The original duck was very confused about being herded into this new yard, but the drake knew exactly where to go!
Now a month on, they all free range together and sleep in the same yard overnight. I've seen them all peacefully wandering about, and have even collected the first two eggs. I still hear the occasional quack during the day, but much less often, and for much shorter duration. And the drake sometimes still chases them, but they seem to be less bothered about it.
I didn't get any complaint from the other neighbour--we are pretty sure his bedroom is at the front of the house, like mine (I can't hear them at all). And nothing from the first neighbour either, so hopefully everyone is sleeping fine.