  We have formed a complete sample of 26 low redshift (z < 0.3) giant radio
galaxies (GRGs) from the WENSS survey, selected at flux densities above 1 Jy at
325 MHz. We present 10.5-GHz observations with the 100-m Effelsberg telescope
of 18 sources in this sample. These observations, together with similar data of
the remaining eight sources, are combined with data from the WENSS, NVSS and
GB6 surveys to study the radio properties of the lobes of these sources at
arcminute resolution. We investigate radio source asymmetries, equipartition
energy densities in the lobes, the presence of lobe pressure evolution with
redshift, the spectral age and the density of the environments of these
sources. We find that the armlength asymmetries of GRGs are slightly larger
than those of smaller sized 3CR radio galaxies and that these are difficult to
explain as arising from orientation effects only. We also find indications that
the lobes of the GRGs, despite their large sizes, are still overpressured with
respect to their environment. Further, we argue that any evolution of lobe
pressure with redshift in these large sources is due to selection effects. A
spectral ageing analysis suggests that the GRGs in our sample are the oldest
members of the group of relatively high power radio sources whose radio powers
have evolved to their currently observed lower values.
