We have used slow positron beam based positron lifetime spectroscopy to study positron diffusion in a thick epitaxial n-type 6H-SiC. The layer is considerably thicker than the maximum positron penetration depth, and can therefore be treated as homogeneous semi-infinite bulk material in an analysis including the time-dependent diffusion of a single group of probe particles. Temperature dependent measurements show a reduction in the positron diffusivity below 100K, which can be interpreted by an increase in trapping to shallow defect states. Above this temperature, the behaviour of the diffusivity is consistent with the expected T(-1/2) dependence due to acoustic phonon scattering.