Thanks Alan but I am having problems with this :/
I can see that the two angles labelled theta in the first my pic are equal.
and
i can see using the second theta in my pic that
sin(theta)=weight down slope/mg so
weight down slope = mg*sin(theta)
BUT
If I use the same angle which you have labeled alpha then
sin(alpha)=mg/weight down the slope
weight down the slope = mg/sin(alpha)
Since I can see that these 2 angles are indeed equal I am totally confused by these 2 different outcomes. 
What is wrong with my logic :/