  We present transmission spectroscopy of the hot-Jupiter WASP-31b using FORS2
on the VLT during two primary transits. The observations cover a wavelength
range of $\approx$400-840nm. The light curves are corrupted by significant
systematics, but these were to first order invariant with wavelength and could
be removed using a common-mode correction derived from the white light curves.
We reach a precision in the transit depth of $\approx$140 ppm in 15 nm bins,
although the precision varies significantly over the wavelength range. Our
FORS2 observations confirm the cloud-deck previously inferred using HST/STIS.
We also re-analyse the HST/STIS data using a Gaussian process model, finding
excellent agreement with earlier measurements. We reproduce the Rayleigh
scattering signature at short wavelengths ($\lesssim$5300 $\AA$) and the
cloud-deck at longer wavelengths. However, our FORS2 observations appear to
rule out the large potassium feature previously detected using STIS, yet it is
recovered from the HST/STIS data, although with reduced amplitude and
significance ($\approx$2.5$\sigma$). The discrepancy between our results and
the earlier STIS detection of potassium ($\approx$4.3$\sigma$) is either a
result of telluric contamination of the ground-based observations, or an
underestimate of the uncertainties for narrow-band features in HST/STIS when
using linear basis models to account for the systematics. Our results further
demonstrate the use of ground-based multi-object spectrographs for the study of
exoplanet atmospheres, and highlight the need for caution in our interpretation
of narrow-band features in low-resolution spectra of hot-Jupiters.
