Because of their unique structural and magnetic properties, ferrofluid suspensions consisting of barium hexaferrite nanoplatelets have been in the spotlight of intensive research. Analysis of the dynamic magnetic susceptibility spectra can elucidate both of these properties using the magnetic-field dependence of the Brownian relaxation mechanism. Applying a small probe field and uniform static bias fields in different configurations, the effect of single particles and collective behaviour is distinctly separated. The importance of considering magnetic interparticle interactions is illustrated by comparing models neglecting or considering such interactions, highlighting limitations considering particle concentration, interaction and excitation-field strengths.