The P30(p,γ)S31 reaction rate is expected to be the principal determinant for the endpoint of nucleosynthesis in classical novae. To date, the reaction rate has only been estimated through Hauser-Feschbach calculations and is unmeasured experimentally. This paper aims to remedy this situation. Excited states in S31 and P31 were populated in the C12(Ne20,n) and C12(Ne20,p) reactions, respectively, at a beam energy of 32 MeV, and their resulting γdecay was detected with the Gammasphere array. Around half the relevant proton unbound states in S31 corresponding to the Gamow window for the P30(p,γ)S31 reaction were identified. The properties of the unobserved states were inferred from mirror symmetry using our extended data on P31. The implications of this new spectroscopic information for the P30(p,γ)S31 reaction rate are considered and recommendations for future work with radioactive beams are discussed.