removing internal structure

Can anyone help me in how to remove all the messy inside of this file?
I want a clean file with just external wall and an internal wall with no messy bits in between.. is this possible?
When i open in blender and go inside the shape, the inside is a mess.. I want to clean it up for better printing.
Thanks in advance for reading this.

Open the file in Netfabb
Click Repair. You have 28 shells
Hover over the icons at the top, Pick Select shells - pick the model, it will turn green.
Pick the icon 'Delete selected triangles' or just press delete on your keyboard.
You are left with 23 Shells.
Go to edit - Undo repairs - click Yes.
I did this so you can check inside the model.

Do the same repair sequence again, once the main shell is selected. Go to edit - toggle selection. the main shell will turn blue.
You now have the other 23 shells selected. Delete them. You now have only one shell.
Apply repair - Remove old part
Go to Part - Export Part - as Stl.

The model is still a mess inside. Shapeways ignores all of the internal geometry. In 3D printing only the outside face/shell is wanted.

Thanks JACANT, another awesome bit of help from you