Yes there are resident hammerheads at Elphinstone north plateau, usually you can see easily them between June and October.
The main problem with Elphinstone it that it is most of the time overcrowded with sometimes 15 or more liveaboard, not counting Zodiacs from Marsa Shagra and other daily boats, so that means 300 or more divers almost at the same time !
Marsa Shagra, a famous dive camp (accomodation goes from tent on the beach to superior bungalows) just in front of Elphinstone, with unlimited house reef diving is a good alternative to liveaboard, because they send Zodiacs everyday day (meeting at 6 AM) and you try to dive before the liveaboards.