  We present a Chandra and XMM-Newton spectral analysis of the nuclei of the
radio galaxies and radio-loud quasars from the 3CRR sample in the redshift
range 0.1 < z < 0.5. In the range of radio luminosity sampled by these objects,
mostly FRIIs, it has been clear for some time that a population of radio
galaxies (`low-excitation radio galaxies') cannot easily participate in models
that unify narrow-line radio galaxies and broad-line objects. We show that
low-excitation and narrow-line radio galaxies have systematically different
nuclear X-ray properties: while narrow-line radio galaxies universally show a
heavily absorbed nuclear X-ray component, such a heavily absorbed component is
rarely found in sources classed as low-excitation objects. Combining our data
with the results of our earlier work on the z<0.1 3CRR sources, we discuss the
implications of this result for unified models, for the origins of mid-infrared
emission from radio sources, and for the nature of the apparent FRI/FRII
dichotomy in the X-ray. The lack of direct evidence for accretion-related X-ray
emission in FRII LERGs leads us to argue that there is a strong possibility
that some, or most, FRII LERGs accrete in a radiatively inefficient mode.
However, our results are also consistent with a model in which the accretion
mode is the same for low- and high-excitation FRIIs, with the lower accretion
luminosities in FRII LERGs attributed instead to more efficient radio
luminosity production in those objects.
