We consider a new physics model with an additional heavy Higgs boson that decays into a top-antitop pair. Its presence induces distortions in the distribution in the invariant mass of the top-antitop system in a neighbourhood of the mass of the heavy Higgs. In order to reliably assess the size of these deviations, we have developed a framework to compute all relevant corrections up to next-to-leading order (NLO) accuracy. This resulted in a code which combines the features of the NLO frameworks Helac and OpenLoops. We present some predictions for this process and, based on those, some preliminary studies of the sensitivity to the parameters of this model at the LHC and the HL-LHC.