  We present an analysis of the evolution of 8625 Luminous Red Galaxies between
z = 0.4 and z = 0.8 in the 2dF and SDSS LRG and QSO (2SLAQ) survey. The LRGs
are split into redshift bins and the evolution of both the luminosity and
stellar mass function with redshift is considered and compared to the
assumptions of a passive evolution scenario. We draw attention to several
sources of systematic error that could bias the evolutionary predictions made
in this paper. While the inferred evolution is found to be relatively
unaffected by the exact choice of spectral evolution model used to compute K+e
corrections, we conclude that photometric errors could be a source of
significant bias in colour-selected samples such as this, in particular when
using parametric maximum likelihood based estimators. We find that the
evolution of the most massive LRGs is consistent with the assumptions of
passive evolution and that the stellar mass assembly of the LRGs is largely
complete by z ~ 0.8. Our findings suggest that massive galaxies with stellar
masses above 10^11 solar masses must have undergone merging and star formation
processes at a very early stage (z > 1). This supports the emerging picture of
downsizing in both the star formation as well as the mass assembly of early
type galaxies. Given that our spectroscopic sample covers an unprecedentedly
large volume and probes the most massive end of the galaxy mass function, we
find that these observational results present a significant challenge for many
current models of galaxy formation
