Start with a view perpendicular to the line you wish to cut, in other words the diagonal is in the middle of the view. Select the plane which is parallel to the view, then on the menu pick Insert->Reference Geometry-.Plane then click on the middle line of the model which will create a plane at that point. Then just draw a 45deg line coincident with both side of the model and make each end the appropriate distance from the bottom or top of the model as you see fit. If it is not yet fully defined, then tie it's midpoint to the origin with a construction line. You may when doing the extruded cut with a simple line be asked which body to keep, if you prefer to avoid that then do a convert entities around the model and trim the parts you don't want, which will give you a clean cut-extrude.
Works like magic! Thanks a lot