  We analyse C$^{18}$O ($J=3-$2) data from a sample of 99 infrared-bright
massive young stellar objects (MYSOs) and compact HII regions that were
identified as potential molecular-outflow sources in the Red MSX source (RMS)
survey. We extract a distance limited (D $<$ 6 kpc) sample shown to be
representative of star formation covering the transition between the source
types. At the spatial resolution probed, Larson-like relationships are found
for these cores, though the alternative explanation, that Larson's relations
arise where surface-density-limited samples are considered, is also consistent
with our data. There are no significant differences found between source
properties for the MYSOs and HII regions, suggesting that the core properties
are established prior to the formation of massive stars, which subsequently
have little impact at the later evolutionary stages investigated. There is a
strong correlation between dust-continuum and C$^{18}$O-gas masses, supporting
the interpretation that both trace the same material in these IR-bright
sources. A clear linear relationship is seen between the independently
established core masses and luminosities. The position of MYSOs and compact HII
regions in the mass-luminosity plane is consistent with the luminosity expected
a cluster of protostars when using a $\sim$40 percent star-formation efficiency
and indicates that they are at a similar evolutionary stage, near the end of
the accretion phase.
