  The completed 2dF QSO Redshift (2QZ) Survey has been used to search for
extreme large-scale cosmological structure (around 200 Mpc) over the redshift
range 0<z<2.5. We demonstrate that statistically significant overdensities and
underdensities do exist and hence represent the detection of cosmological
fluctuations on comoving scales that correspond to those presently detected in
the cosmic microwave background. However, the fractional overdensities on
scales >100Mpc are in the linear or only weakly non-linear regime and do not
represent collapsed non-linear structures. We compare the measurements with the
expectation of a standard LCDM model by measuring the variance of counts in
cells and find that, provided the distribution of QSOs on large scales exhibits
a mild bias with respect to the distribution of dark matter, the observed
fluctuations are found to be in good agreement with the model. There is no
evidence on such scales for any extreme structures that might require, for
example, departures from the assumption of Gaussian initial perturbations. Thus
the power-spectrum derived from the 2QZ Survey appears to provide a complete
description of the distribution of QSOs. The amount of bias and its redshift
dependence that is required is consistent with that found from studying the
clustering of 2QZ QSOs on 10 Mpc scales, and may be adequately described by an
approximately redshift-invariant power spectrum with normalisation sigma_8=1.0
corresponding to a bias at z=0 of b=1.1 rising to b=2 at the survey's mean
redshift z=1.5.
