A linear perturbation theory is used to model the MHD stability of tokamak equilibria under the application of external 3D magnetic perturbations [C.C. Hegna, Physics of Plasmas 21 :072502, 2014]. The symmetry breaking produces the coupling of toroidal n modes. We use ELITE [H.R. Wilson et al., Physics of Plasmas 9 :1277, 2002] to produce both a linearly perturbed non-axisymmetric equilibrium state as well as the linear axisymmetric modes, that are coupled for the stability analysis. The symmetry breaking produces coupling of modes with different toroidal mode number n and poloidal localisation of the non-axisymmetric peeling-ballooning mode is observed in comparison to the axisymmetric case.