  Air showers recorded by the Haverah Park Array during the years 1974 - 1987
have been re-analysed. For the original estimate of the energy spectrum, a
relationship between the ground parameter $\rho$(600) and the primary energy,
as determined by Hillas in the 1970s, was used. Here we describe the energy
spectrum obtained using the QGSJET98 interaction model in the CORSIKA Monte
Carlo code, together with GEANT to simulate the detailed detector response to
ground particles. A new energy spectrum in the range 3 $\times$ 10$^{17}$ eV to
4 $\times$ 10$^{18}$ eV is presented.
