Variation in the redshift of the peak space density with radio power for the best-fitting steep spectrum grid determined here (black line), compared with that predicted using the simple model described in Sect. 5.2 (blue line). Dotted lines delineate the relative contributions to the latter from the two underlying populations. The error bars show the uncertainty in the polynomial fits used to determine the peak position, whilst the shaded region represents the range in results found from varying the input parameters used in the RLF grid modelling.