Neutron energy spectra from deuterium neutral beam heated plasmas at the Joint European Torus (JET) have been measured with a time-of-flight spectrometer. The different source fractions, resulting from thermal-thermal (tt), beam-thermal (bt) and beam-beam (bb) interactions have been extracted. The measured tt fraction and associated ion temperature is checked for consistency with diagnostic data for electron density ne, effective charge Zeff, ion temperature Ti and total neutron emission. Two different ways of comparing the data are presented. Firstly, the measured tt fraction, total neutron emission, ne, Zeff and Ti profile shape are used to deduce a consistent Ti, which is compared with Ti values obtained independently by other diagnostics. Secondly, the measured tt fraction, total neutron emission, ne profile shape and Ti are used to determine a consistent thermal deuterium density no, which is compared with no obtained from Zeff measurements. The experimental errors from the analysis of the neutron spectra can be reduced by carefully restricting the analysis to plasmas within well defined ranges of ion temperature and tt fraction. The values of Ti and no thus derived are in good agreement with the values obtained independently. The method demonstrated in this paper can therefore be used with confidence to obtain plasma parameters under conditions which lie outside the operating range of the more conventional diagnostics, provided these conditions are met.