  Geometric modelling of Coronal Mass Ejections (CMEs) is a widely used tool
for assessing their kinematic evolution. Furthermore, techniques based on
geometric modelling, such as ELEvoHI, are being developed into forecast tools
for space weather prediction. These models assume that solar wind structure
does not affect the evolution of the CME, which is an unquantified source of
uncertainty. We use a large number of Cone CME simulations with the HUXt solar
wind model to quantify the scale of uncertainty introduced into geometric
modelling and the ELEvoHI CME arrival times by solar wind structure. We produce
a database of simulations, representing an average, a fast, and an extreme CME
scenario, each independently propagating through 100 different ambient solar
wind environments. Synthetic heliospheric imager observations of these
simulations are then used with a range of geometric models to estimate the CME
kinematics. The errors of geometric modelling depend on the location of the
observer, but do not seem to depend on the CME scenario. In general, geometric
models are biased towards predicting CME apex distances that are larger than
the true value. For these CME scenarios, geometric modelling errors are
minimised for an observer in the L5 region. Furthermore, geometric modelling
errors increase with the level of solar wind structure in the path of the CME.
The ELEvoHI arrival time errors are minimised for an observer in the L5 region,
with mean absolute arrival time errors of $8.2\pm1.2$~h, $8.3\pm1.0$~h, and
$5.8\pm0.9$~h for the average, fast, and extreme CME scenarios.
