  Continuum reverberation mapping of AGN can provide new insight into the
nature and geometry of the accretion flow. Some of the X-rays from the central
corona irradiating the disc are absorbed, increasing the local disc
temperature. This gives an additional re-processed contribution to the spectral
energy distribution (SED) which is lagged and smeared relative to the driving
X-ray light-curve. We directly calculate this reverberation from the accretion
disc, creating fully time dependent SEDs for a given X-ray light-curve. We
apply this to recent intensive monitoring data on Fairall 9, and find that it
is not possible to produce the observed UV variability by X-ray reprocessing of
the observed light-curve from the disc. Instead, we find that the majority of
the variability must be intrinsic to the UV emission process, adding to
evidence from changing look AGN that this region has a structure which is quite
unlike a Shakura-Sunyaev disc. We filter out this long timescale variability
and find that reprocessing alone is still insufficient to explain even the fast
variability in our assumed geometry of a central source illuminating a flat
disc. The amplitude of reprocessing can be increased by any vertical structure
such as the BLR and/or an inner disc wind, giving a better match. Fundamentally
though the model is missing the major contributor to the variability, intrinsic
to the UV/EUV emission rather than arising from reprocessing.
