This paper investigates the numerical modelling of VLF Trimpis produced by a D region inhomogeneity on the Great Circle Path . Two different codes are used. The first is a 2D finite element method (FEM) code , whose solutions are rigorous and valid in the strong scattering or non-Born limit. The second code is a fully 3D model that invokes the Born approximation. The predicted Trimpis from these codes compare closely, thus confirming the validity of both models. The modal scattering matrices have a comparable structure, and indicate strong scattering between the dominant TM modes. Analysis of the scattering matrix from the FEM code delineates the Born regime. For a LIE with a radius of 100kms, the Born approximation becomes invalid at an electron density perturbation of about 8 el/cc.