Hi Finearcltd, you can use the Edit > Trim command to cut a surface with curves. The Trim command also has planar curve projection built into it already, so you don't need to do the projection as an extra separate step, just use the original 2D source curve that you were projecting as the cutting object in the Trim command.

There's a description of Trim here:
http://moi3d.com/3.0/docs/moi_command_reference4.htm#trim

- Michael