  We present K-band imaging of all 49 radio galaxies in the 7C-I and 7C-II
regions of the 7C Redshift Survey (7CRS). The low-frequency (151 MHz) selected
7CRS sample contains all sources with flux-densities S_151 > 0.5 Jy in three
regions of the sky. We combine the K-band magnitudes of the 7CRS radio galaxies
with those from the 3CRR, 6CE and 6C* samples to investigate the nature of the
relationship between K-magnitude and redshift and whether there is any
dependence upon radio luminosity. We find that radio galaxies appear to belong
to a homogeneous population which formed the bulk of their stars at high
redshifts (z_f>5) and evolved passively from then until they reach a mean
present-day luminosity of 3L*. We find a significant difference between the
K-magnitudes of the 7CRS and 3CRR radio galaxies with the 7CRS galaxies being
\~0.55 mag fainter at all redshifts. The cause of this weak correlation between
stellar and radio luminosities probably lies in mutual correlations of these
properties with the central black hole mass. We compare the evolution-corrected
host luminosities at a constant radio luminosity and find that the typical host
luminosity (mass) increases by approximately 1L* from z~2 to z~0.5 which,
although a much smaller factor than predicted by semi-analytic models of galaxy
formation, is in line with results on optically-selected quasars. Our study has
therefore revealed that the small dispersion in stellar luminosity of radio
galaxies around 3L* includes subtle but significant differences between the
host galaxies of extreme- and moderate-power radio sources at fixed redshift,
and between those of high- and low-redshift radio sources at fixed radio
luminosity.
