It is possible to grow decent yields with much less N than is the convention, but a serious long term approach to soil health, microbial, fungal and insect life needs to be focused on. Large amounts of ammonium nitrate is the main thing stopping conventional farming from cutting down on inputs and improving the soil ecology. I firmly believe that over use of nitrogen drives all our weed, disease and pest issues on farm.Interesting.
Are you using FYM or other soil amendment?
I would have saved £360 Ha on WW and £180 Ha on SW on those inputs.
It has been proven that 200kg N doubles your yield compared with no N, and that would be in trials where there was N applied the previous year, or it was in a legume. Organic production would be about half conventional yields.
I'm not sure what your average yields were in those years, but I'm not sure I would have got 7t/ha milling spec WW with zero N, or 6.35 t/ha SW - but perhaps I should try a small field or a tramline. Might work best with spring barley. The only input I might have to apply in some fields would be Mn.
A good trial would be only herbicides; herbicides and half other inputs; and both compared to normal treatment.