Top: selection function adopted for the present study. The probability of detecting a cluster as C1 is given by the isocontours as a function of count-rate and core-radius. This map has been derived from extensive XMM image simulations and the two axes represent the true (input) cluster parameters; it is thus only valid for the conditions (exposure time and background) under which the simulations were run. Bottom: resulting cluster redshift distribution for 100 deg² assuming the set of physical and cosmological parameters given in Tables 1 and 4; the cluster density is of the order of 6/deg².