  This paper presents a new grid-based method for investigating the evolution
of the steep-spectrum radio luminosity function, with the aim of quantifying
the high-redshift cut-off suggested by previous work. To achieve this, the
Combined EIS-NVSS Survey of Radio Sources (CENSORS) has been developed; this is
a 1.4 GHz radio survey, containing 135 sources complete to a flux density of
7.2 mJy, selected from the NRAO VLA Sky Survey (NVSS) over 6 deg^2 of the ESO
Imaging Survey (EIS) Patch D. The sample is currently 73% spectroscopically
complete, with the remaining redshifts estimated via the K-z or I-z
magnitude-redshift relation. CENSORS is combined with additional radio data
from the Parkes All-Sky, Parkes Selected Regions, Hercules and VLA COSMOS
samples to provide comprehensive coverage of the radio power vs. redshift
plane. The redshift distributions of these samples, together with radio source
count determinations, and measurements of the local luminosity function,
provide the input to the fitting process. The modelling reveals clear declines,
at > 3sigma significance, in comoving density at z > 0.7 for lower luminosity
sources (log P = 25-26); these turnovers are still present at log P > 27, but
move to z > 3, suggesting a luminosity-dependent evolution of the redshift
turnover, similar to the `cosmic downsizing' seen for other AGN populations.
These results are shown to be robust to the estimated redshift errors and to
increases in the spectral index for the highest redshift sources. Analytic fits
to the best-fitting steep spectrum grid are provided so that the results
presented here can be easily accessed by the reader, as well as allowing
plausible extrapolations outside of the regions covered by the input datasets
