ok thanks for the info guys.
I must admit I thought hammers and reef shark were pretty much guaranteed as long as you were prepared to get up early enough and travel to the right spot.
Rasdhoo Atoll Divers still offer hammerhead dives on their website :
http://www.rasdhoodivers.com/
so it may be worth dropping them a line to see what the frequency of sitings are
There are some famous shark points: Fish head and Maaya thila in North Ari Atoll. These were made famous years ago by the shark feeding antics of some well known photographers such as Herwarth Voigtmann. Common species here are Grey and White tip reef sharks. The island of Maayafushi, as the name suggests, is conveniently located near Maaya thila.
Mantas on the other hand, pretty much are guarranteed if you are staying in South Ari, as Tim did, during the right period.
The challenge is that the hammerhead sites, reef shark sites and manta points are all spread out, so it's difficult to find a perfect island to get them all - The further away the island from the site, the less chance there is that they will dive it frequently. Alex's suggestion of a liveaboard is certainly the best option for some hardcore diving. You could do a week on an island then a week on a liveaboard if you wanted to mix and match, but you would not cover the ground or get the variety of sites that you would with a 2 week liveaboard trip.
If you do choose an island, check in advance what sites they cover and the frequency of the visits to any known hot sites
Steve