  Lyman-alpha forest data probing the post-reionization Universe shows
surprisingly large opacity fluctuations over rather large ($\ge$50 comoving
Mpc/h) spatial scales. We model these fluctuations using a hybrid approach
utilizing the large volume Millennium simulation to predict the spatial
distribution of QSOs combined with smaller scale full hydrodynamical simulation
performed with RAMSES and post-processed with the radiative transfer code ATON.
We produce realictic mock absorption spectra that account for the contribution
of galaxies and QSOs to the ionising UV background. This improved models
confirm our earlier findings that a significant ($\ge$50%) contribution of
ionising photons from QSOs can explain the large reported opacity fluctuations
on large scales. The inferred QSO luminosity function is thereby consistent
with recent estimates of the space density of QSOs at this redshift. Our
simulations still somewhat struggle, however, to reproduce the very long (110
comoving Mpc/h) high opacity absorption through observed in ULAS J0148+0600,
perhaps suggesting an even later end of reionization than assumed in our
previously favoured model. Medium-deep/medium area QSO surveys as well as
targeted searches for the predicted strong transverse QSO proximity effect
whould illuminate the origin of the observed large scale opacity fluctuations.
They would allow to substantiate whether UV fluctuations due to QSO are indeed
primarily responsible, or whether significant contributions from other recently
proposed mechansims such as large scale fluctuations in temperature and mean
free path (even in the absence of rare bright sources) are required.
