Thank you all for your inputs.
I have to admit that it was the "CFS" explanation that I described as fundamentally flawed. As far as I can see the main reason for designing in a longitudinal dihedral would be to achieve a cruise set-up with the wing at alpha for best TAS/Drag and the tail at minimum drag alpha, to minimise trim drag. This would rely on the wing/body moment at cruise alpha being near zero, achieved by careful CG positioning. By its nature this implies a longitudinal dihedral of about 4deg.
How does this seem to you all?
Dick W