Help, have got nearly to the end of the game and am stuck on the last p…

Help, have got nearly to the end of the game and am stuck on the last part. Have found all the heads and got the stone disc that goes in the middle of the circle. Cannot for the life of me work out what to do next? Am finding it very hard to tell which head is the right one apart from the obious ones, like scream and the one with a headress. The rest was quite easy so am surprised that I am being so stupid with this bit. Any ideas please?