As the former owner of the only post-war Sheffield Simplex for many years, in the fifties, I have to stare that Mr. Ulmann is wrong in stating that the post-war car had OHV. It had side valves.
I found the Sheffield Simplex was quite a little more powerful and speedy than the Rolls-Royce Silver Ghosts of the same period but was prone to boil under hard conditions. This trouble could not be cured, because both the inlet and outlet of the cooling water was at the bottom of the separate cylinders, thus leaving a pocket of non-circulating hot or boiling water at the top of each cylinder.
Celbridge, Co. Kildare