The magnetic excitation spectrum of the randomly diluted ferromagnet, Ni₂Mn₀.₈V₀.₂Sn, has been measured by neutron inelastic techniques. The frequencies are reduced from those observed for the non-diluted ferro-magnet, Ni₂MnSn, by a factor that depends on wavevector. The shape of the frequency shift is characteristic of an in-band resonance. Computer simulation calculations have been performed for large finite arrays (~15,000 spins) including up to third neighbour interactions. The predicted frequencies agree with experiment. The calculations show a resonant effect however it is not as pronounced as the observed effect.