Upper panel: mass function for the cosmological model best fitting the REFLEX Survey data (thick black line). The dark shaded region shows the uncertainties while marginalising over the 1σ limits of the most important cosmological and scaling relation parameters (see text). The lightly shaded region shows in addition the variation with the Hubble constant in the range 67 to 73 km s^-1 Mpc^-1. The data points give the mass function derived from the X-ray luminosity function of the REFLEX cluster survey. The dashed line gives the best fitting mass function for a redshift of z = 0.102 for comparison with the REFLEX data. Lower panel: ratio of the uncertainty limits to the best fitting function, where the light and dark shaded regions have the same meaning as above. The dashed and dashed-dotted lines show the ratio for the best fitting functions for a Hubble constant of 67 and 73 km s^-1 Mpc^-1. The dotted line shows the best fitting function allowing the sum of the mass of the neutrinos to be 0.4 eV.