  We report ground-based transmission spectroscopy of the highly irradiated and
ultra-short period hot-Jupiter WASP-103b covering the wavelength range
$\approx$ 400-600 nm using the FORS2 instrument on the Very Large Telescope.
The light curves show significant time-correlated noise which is mainly
invariant in wavelength and which we model using a Gaussian process. The
precision of our transmission spectrum is improved by applying a common-mode
correction derived from the white light curve, reaching typical uncertainties
in transit depth of $\approx$ 2x10$^{-4}$ in wavelength bins of 15 nm. After
correction for flux contamination from a blended companion star, our
observations reveal a featureless spectrum across the full range of the FORS2
observations and we are unable to confirm the Na absorption previously inferred
using Gemini/GMOS or the strong Rayleigh scattering observed using broad-band
light curves. We performed a Bayesian atmospheric retrieval on the full
optical-infrared transmission spectrum using the additional data from
Gemini/GMOS, HST/WFC3 and Spitzer observations and recover evidence for H$_2$O
absorption at the 4.0$\sigma$ level. However, our observations are not able to
completely rule out the presence of Na, which is found at 2.0$\sigma$ in our
retrievals. This may in part be explained by patchy/inhomogeneous clouds or
hazes damping any absorption features in our FORS2 spectrum, but an inherently
small scale height also makes this feature challenging to probe from the
ground. Our results nonetheless demonstrate the continuing potential of
ground-based observations for investigating exoplanet atmospheres and emphasise
the need for the application of consistent and robust statistical techniques to
low-resolution spectra in the presence of instrumental systematics.
