An alternative model to Kogelnik's coupled wave theory of the volume holographic grating is developed in terms of an infinite array of parallel stacked mirrors. The model is based on summing the individual Fresnel reflections from an infinite number of infinitesimal discontinuities in the permittivity profile. The resulting first-order coupled partial differential equations are solved in a rotated frame of reference in order to derive analytical expressions for the diffraction efficiency of the general slanted grating at an arbitrary angle of incidence. The model has been tested using computational solutions of the Helmholtz equation for the unslanted reflection grating. For index modulations characteristic of modern silver halide and photopolymer materials used in display and optical element holography the new model shows excellent agreement with the numerical results. Kogelnik's model also provides good agreement as long as the dephasing parameter is not too large. The model has been tested against Kogelnik's theory for a variety of cases with finite fringe slant with good agreement for typical index modulations. A further advantage of the new model is that colour holographic gratings may be treated at and away from Bragg resonance. Numerical and analytical results are presented concerning the diffractive efficiency of two- and three-colour holographic gratings.