Mesh Splitting

I have a mesh split challenge!...

I can do this (with no save option) using the netfabb demo software but its €239 to buy the full software so I'm trying to find a way of doing it with Rhino + GH first before conceding and buying the software.

I did manage it once with Rhino but it was messy and it hasn't worked since!

It works in netfabb if I save as .STL files, then repair the files first, so I'm wondering if I need to do some more mesh processing in GH before the split will work in Rhino... i.e. do meshes have to be closed before split operations will work? I've tried unifying normals, triangulating the mesh, breaking it down into the component parts of the mesh,

In the image below I'm trying to cut away the purple meshes from the grey mesh...

Rhino does create split lines on the mesh in the right place but it seems to shatter the mesh and then create loads of overlapping meshes. The meshes don't actually get split along the split lines even though they are there.

Before I start posting definitions etc, does anyone think this should be possible in Rhino + GH and have any tips?