  We present new observations of the transmission spectrum of the hot Jupiter
WASP-6b both from the ground with the Very Large Telescope (VLT) FOcal Reducer
and Spectrograph (FORS2) from 0.45-0.83 $\mu$m, and space with the Transiting
Exoplanet Survey Satellite (TESS) from 0.6-1.0 $\mu$m and the Hubble Space
Telescope (HST) Wide Field Camera 3 from 1.12-1.65 $\mu$m. Archival data from
the HST Space Telescope Imaging Spectrograph (STIS) and Spitzer is also
reanalysed on a common Gaussian process framework, of which the STIS data show
a good overall agreement with the overlapping FORS2 data. We also explore the
effects of stellar heterogeneity on our observations and its resulting
implications towards determining the atmospheric characteristics of WASP-6b.
Independent of our assumptions for the level of stellar heterogeneity we detect
Na I, K I and H$_2$O absorption features and constrain the elemental oxygen
abundance to a value of [O/H] $\simeq -0.9\pm0.3$ relative to solar. In
contrast, we find that the stellar heterogeneity correction can have
significant effects on the retrieved distributions of the [Na/H] and [K/H]
abundances, primarily through its degeneracy with the sloping optical opacity
of scattering haze species within the atmosphere. Our results also show that
despite this presence of haze, WASP-6b remains a favourable object for future
atmospheric characterisation with upcoming missions such as the James Webb
Space Telescope.
