  Comet 67P/Churyumov-Gerasimenko was selected in 2003 as the new target of the
Rosetta mission. It has since been the subject of a detailed campaign of
observations to characterise its nucleus and activity. Here we present
previously unpublished data taken around the start of activity of the comet in
2007/8, before its last perihelion passage. We constrain the time of the start
of activity, and combine this with other data taken throughout the comet's
orbit to make predictions for its likely behaviour during 2014/5 while Rosetta
is operating. A considerable difficulty in observing 67P during the past years
has been its position against crowded fields towards the Galactic centre for
much of the time. The 2007/8 data presented here were particularly difficult,
and the comet will once again be badly placed for Earth-based observations in
2014/5. We make use of the difference image analysis (DIA) technique, which is
commonly used in variable star and exoplanet research, to remove background
sources and extract images of the comet. In addition, we reprocess a large
quantity of archival images of 67P covering its full orbit, to produce a
heliocentric lightcurve. By using consistent reduction, measurement and
calibration techniques we generate a remarkably clean lightcurve, which can be
used to measure a brightness-distance relationship and to predict the future
brightness of the comet. We determine that the comet was active around November
2007, at a pre-perihelion distance from the Sun of 4.3 AU. The comet will reach
this distance, and probably become active again, in March 2014. We find that
the dust brightness can be well described by Af\rho \propto r^-3.2
pre-perihelion and r^-3.4 post-perihelion, and that the comet has a higher
dust-to-gas ratio than average. A model fit to the photometric data suggests
that only a small fraction (1.4%) of the surface is active.
