  We study exclusive vector meson photoproduction, $\gamma p \to V + p$ with
$V=J/\psi$ or $\Upsilon$, at NLO in collinear factorisation, in order to
examine what may be learnt about the gluon distribution at very low $x$. We
examine the factorisation scale dependence of the predictions. We argue that,
using knowledge of the NLO corrections, terms enhanced by a large $\ln(1/\xi)$
can be reabsorbed in the LO part by a choice of the factorisation scale. (In
these exclusive processes $\xi$ takes the role of Bjorken-$x$.) Then, the scale
dependence coming from the remaining NLO contributions has no $\ln(1/\xi)$
enhancements. As a result, we find that predictions for the amplitude of
$\Upsilon$ production are stable to within about $\pm 15\%$. This will allow
data for the exclusive process $p p \to p\Upsilon p$ at the LHC, particularly
from LHCb, to be included in global parton analyses to constrain the gluon PDF
down to $x\sim 10^{-5}$. Moreover, the study of exclusive $J/\psi$
photoproduction indicates that the gluon density found in the recent global PDF
analyses is too small at low $x$ and low scales.
