The large majority of GaN epitaxial films have been grown on substrates which show both lattice and thermal expansion mismatch and, as a result, are significantly strained. The effect is most readily monitored by measuring exciton energies in photoluminescence spectra, and this paper compares published data on free exciton energies in a homoepitaxial film with a range of heteroepitaxial samples grown on sapphire substrates, all the films having the hexagonal (wurtzite) structure. We observe that strain results in a marked increase in separation of the B and C excitons while leaving the A, B separation relatively unaffected, and interpret this in terms of the quasi-cubic band structure model developed by Hopfield for the very similar case of CdS and other II - VI compounds. Within the limits of this model, there is an ambiguity in the interpretation of exciton energies which depends on the fact that the spin - orbit and crystal field splitting parameters are interchangeable. Consideration of strain effects removes this ambiguity and allows us to derive values of meV and meV for the unstrained homoepitaxial sample, consistent with the earlier estimates of Dingle et al based on relative intensity measurements.