A simple 0-D computer code has been prepared for predicting the neutron emission in the initial phase of beam-heated hot-ion H-mode discharges in the Joint European Torus (JET). It performs very well for discharges with low particle recycling up to the onset of MHD-related activity, which usually occurs within one second of commencing full power heating. It can also be used to model the entire discharge, including the termination, when fuelling of the plasma core with cold neutrals is adopted as a first-order approximation for edge recycling effects. Specifically, given the time traces for the beam heating power, the global neutron emission, average electron density, Zeff and Dα signals, the code can satisfactorily reconstruct the core volume rotation rate, ion temperature and electron temperature for the entire duration of the discharge, and it also provides an explanation for the anomalously low burnup of d-d fusion product tritons that is sometimes observed. The code provides insight into the relative intensities of the three contributing reaction mechanisms to the total neutron emission (thermal, beam-thermal and beam-beam reactions) as an aid to interpretation of data from the various neutron diagnostics on JET, including the neutron profile monitor and the neutron spectrometers. Tritium fuelling and beam injection are also implemented, so the code can be used to extrapolate the neutron emission from existing d-d discharges to that expected from similar d-t discharges