  We use three samples (3CRR, 6CE and 6C*) to investigate the radio luminosity
function (RLF) for the `most powerful' low-frequency selected radio sources. We
find that the data are well fitted by a model with a constant co-moving space
density at high redshift as well as by one with a declining co-moving space
density above some particular redshift. This behaviour is very similar to that
inferred for steep-spectrum radio quasars by Willott et al (1998) in line with
the expectations of Unified Schemes. We conclude that there is as yet no
evidence for a `redshift cut-off' in the co-moving space densities of powerful
classical double radio sources, and rule out a cut-off at z < 2.5.
