We have investigated the onset of convection instability in a heterogeneous inclined porous layer. Our equations also account for local thermal non-equilibrium. We modelled the effect of heterogeneity by assuming that the layer is composed of two porous sub-layers with different properties, such as permeability, fluid conductivity, solid conductivity, interphase heat transfer coefficient and porosity. We identified which of these factors have major and which have minor effect on the instability. We also characterized the accuracy of one-term Galerkin approximation for this problem. In order to do this, we compared the results obtained by Galerkin approximation with the results obtained by a highly accurate numerical solver.