  WD 1145+017 is currently the only white dwarf known to exhibit periodic
transits of planetary debris as well as absorption lines from circumstellar
gas. We present the first simultaneous fast optical spectrophotometry and
broad-band photometry of the system, obtained with the Gran Telescopio Canarias
(GTC) and the Liverpool Telescope (LT), respectively. The observations spanned
$5.5$ h, somewhat longer than the $4.5$-h orbital period of the debris.
Dividing the GTC spectrophotometry into five wavelength bands reveals no
significant colour differences, confirming grey transits in the optical. We
argue that absorption by an optically thick structure is a plausible
alternative explanation for the achromatic nature of the transits that can
allow the presence of small-sized ($\sim\mu$m) particles. The longest ($87$
min) and deepest ($50$ per cent attenuation) transit recorded in our data
exhibits a complex structure around minimum light that can be well modelled by
multiple overlapping dust clouds. The strongest circumstellar absorption line,
Fe II $\lambda$5169, significantly weakens during this transit, with its
equivalent width reducing from a mean out-of-transit value of $2$ \AA\ to $1$
\AA\ in-transit, supporting spatial correlation between the circumstellar gas
and dust. Finally, we made use of the Gaia Data Release 2 and archival
photometry to determine the white dwarf parameters. Adopting a helium-dominated
atmosphere containing traces of hydrogen and metals, and a reddening
$E(B-V)=0.01$ we find $T_\mathrm{eff}=15\,020 \pm 520$ K, $\log g=8.07\pm0.07$,
corresponding to $M_\mathrm{WD}=0.63\pm0.05\ \mbox{$\mathrm{M}_{\odot}$}$ and a
cooling age of $224\pm30$ Myr.
