Lost face on a complex survey document

I have a detailed survey which has lost its face, hence I need to replace it. When I try to do this all the geometry is now one face and cant be individually seperated to colour etc. Can you advise?
Many thanks in advance
Analysis sheet rushgrove.skp (9.4 MB)

You need to close all those gaps (and clean up the model) before you can create faces

Thank you for the reply