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, =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] ; Comment: 21 pages, 18 figures, MN style, uses psfig. Accepted for publication in MN