3D Mesh - Culling Surface

I've created a 3D mesh using the marching cubes algorithm.  The mesh looks fine but I'm having problems getting the transparency and lighting to look good.  I think I know why but I don't know how to fix it:

When I use the Draw Style:Face Culling Mode property, if I select 'Cull Front Face' it culls what I consider to be the back face and visa versa.  

How do I control what is the front and what is the back face please?  Is it an ordering issue?

Thanks in advance,

Martin