We present long-slit, optical spectra of the merging system Arp220, obtained using the William Herschel Telescope (WHT) on La Palma. These data were taken as a part of a large study of ultraluminous infrared galaxies (ULIRGs) with the aim of investigating the evolution and star formation histories of such objects. Spectral synthesis modelling has been used to estimate the ages of the stellar populations found in the diffuse light sampled by the spectra. As the closest ULIRG, it proved possible to perform a detailed study of the stellar populations over the entire body of the object. The data show a remarkable uniformity in the stellar populations across the full 65 arcsec covered by our slit positions, sampling the measurable extent of the galaxy. The results are consistent with a dominant intermediate-age stellar population (ISP) with age 0.5 <= tISP <= 0.9 Gyr that is present at all locations, with varying contributions from a young (<=0.1 Gyr) stellar population (YSP) component. However, it is notable that while the flux contribution of the YSP component in the extended regions is relatively small (<=40 per cent), adequate fits in the nuclear region are only found for combinations with a significant contribution of a YSP component (22-63 per cent). Moreover, while a low intrinsic reddening (E(B - V) <~ 0.3) is found for the ISPs in the extended regions, intrinsic reddening values as high as E(B - V) ~ 1.0 are required in the galactic centre. This clearly reflects the presence of a reddening gradient, with higher concentrations of gas and dust towards the nuclear regions, coinciding with dust lanes in the Hubble Space Telescope images. Overall, our results are consistent with models that predict an epoch of enhanced star formation coinciding with the first pass of the merging nuclei (represented by the ISP), with a further episode of star formation occurring as the nuclei finally merge together (represented by the YSP and ULIRG).