How can I show due diligence when assembling products and systems from CE marked sub-assemblies or apparatus?

It is difficult to offer general guidance here as different measures will be needed for the wide variety of possible situations. A computer system assembled from finished products (i.e. keyboard, mouse, monitor and completed box containing boards, disc drives and power supply) may need no further assessment; there is only one way these can be interconnected and they will have been assessed as completed products by their manufacturers. A machine consisting of PLCs, motor drives, encoders, sensors, etc. may require a full assessment.