structure.\
In addition, we investigate the host galaxy environment in another way, through ESO-VLT/FORS low- and medium-resolution spectroscopy, as well as optical-to-X-ray spectral energy distribution (SED) modelling. Ly$\alpha$ and metal absorption systems, often observed in GRB lines of sight, can be used to derive physical properties of the absorbing gas clouds, such as kinematics, densities and metallicities [@Vreeswijk06; @Ledoux09]. On the other hand, the SED provides information on both the dust inside the host galaxy and the spectral properties of the GRB afterglow itself [e.g., @Starling07]. Overall, interpreting the combined optical and X-ray spectra and light curves within the context of the fireball model can provide a probe of the blast-wave physics, as well as the GRB environment [Sari, Piran & Narayan 1998; @Zhang06].\
The paper is organized as follows: observations and data reduction are presented in Sec. 2, while their analysis is reported in Sec. 3: first the ULTRASPEC light curve, then the optical spectroscopy, the SED modelling and finally the optical and X-ray afterglow temporal decay. We discuss our results in Sec. 4 and summarize them in the last section. Throughout the paper we use the convention $F_\nu \left(t\right) \propto t^{-\alpha}\nu^{-\beta}$ for the flux density, where $\alpha$ is 