Cracked roof tile, can I replace it from inside?

Have got a damp problem in bedroom which seems to be getting worse, it seems to appear on the sloping part of the ceiling right the way across and the walls are getting slightly damp. Am trying to find the cause and have looked in the attic and found a cracked tile (not sure if this is the whole problem as I mentioned the damp seems to be in a much wider area than below the cracked tile). Anyhow, is it possible for me to replace the tile from inside the attic without the need to go onto the roof? The tiles seem to simply latch over slats going across the roof.
With regards to the bigger problem am going to try sealing the outside walls first and see if that helps too. Not sure about the facias (dont really know anything about them to be honest).
Any advice or help would be greatly appreciated, as I said it seems to be getting quite bad and is definately worse when we have wet weather (which is too often in this country :) )
thanks
Paul