  We present a deep UBI CCD survey using the Palomar 5-m telescope of a sample
of high X-ray luminosity, distant clusters selected from the ROSAT All-Sky
Survey. The 10 clusters lie at z=0.22-0.28, an era where evolutionary effects
have been reported in the properties of cluster galaxy populations. Our
clusters thus provide a well-defined sample of the most massive systems at
these redshifts to quantify the extent and variability of these evolutionary
effects. Moreover, by concentrating on a narrow redshift range, we can take
advantage of the homogeneity of our sample to combine the catalogues from all
the clusters to analyse the bulk properties of their populations. The core
regions of these clusters ontain only a small proportion of star-forming
galaxies, and they therefore do not exhibit a `Butcher-Oemler' effect. Focusing
on the redder cluster galaxies we find that their integrated luminosity is well
correlated with the cluster X-ray temperatures, and hence with cluster mass.
Furthermore, the typical UV-optical colours of the elliptical sequences in the
clusters exhibit a small cluster-to-cluster scatter, <=2%, indicating that
these galaxies are highly homogeneous between clusters. However, at fainter
magnitudes we observe an increase in the range of mid-UV colours of galaxies
possessing strong 4000A breaks. In the light of the apparent decline in the S0
populations of z>=0.4 clusters (Dressler et al. 1997), and in view of the
luminosities and colours of this population, we propose that they may be the
progenitors of the S0 population of local rich clusters, caught in the final
stage before they become completely quiescent. [abridged]
