I have found a way of producing the desired effect ( though only for a straight 45 degree profile ) by drawing two 'L' shaped plans, the bottom one narrower than the top, then lofting between them.
I am limited by my inexperience I suspect, because i can't find a way of extruding the ends of the lofted cornice to fit my unit.
in the end I went into top view and drew the pre-loft shapes directly onto the top of the unit ( as a template ) then went into front view and lifted the larger one before lofting.
The end result is good but I can only curve the profile by adjusting individually the cv's and that only by guess work and by eye to judge that the same changes are made at each necessary point on the cornice ( each end and twice at the middle )
I have previously extruded curves to produce curved profile sections for ceiling cornices. They render beautifully and the intersection point just doesn't matter because it is hidden behind the walls.
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