  We present unequivocal evidence for a huge (~80 x 100 x 100 Mpc^3)
super-structure at redshift z=0.27 in the 3-D distribution of radio galaxies
from the TONS08 sample, confirming tentative evidence for such a structure from
the 7C redshift survey (7CRS). A second, newly discovered super-structure is
also less securely found at redshift 0.35 (of dimensions ~100 x 100 x 100
Mpc^3). We present full observational details on the TONS08 sample which was
constructed to probe structures in the redshift range 0 < z < 0.5 by matching
NVSS sources with objects in APM catalogues to obtain a sample of optically
bright (E ~ R < 19.83), radio faint (1.4-GHz flux density S_1.4 > 3 mJy) radio
galaxies in the same 25 deg^2 area as part-II of the 7CRS. Out of the total
sample size of 84 radio galaxies, at least 25 are associated with the two ~100
Mpc-scale super-structures. We use quasi-linear structure formation theory to
estimate the number of such structures expected in the TONS08 volume if the
canonical value for radio galaxy bias is assumed. Under this assumption, the
structures represent ~ 4-5 sigma peaks in the primordial density field and
their expected number is low (~10^-2 - 10^-4). Fortunately, there are several
plausible explanations (many of which are testable) for these low probabilities
in the form of potential mechanisms for boosting the bias on large scales.
These include: the association of radio galaxies with highly biased rich
clusters in super-structures, enhanced triggering by group/group mergers, and
enhanced triggering and/or redshift space distortion in collapsing systems as
the growth of super-structures moves into the non-linear regime (abridged).
