Data from forced Vertical Displacement Event (VDE) experiments in the Mega Ampere Spherical Tokamak (MAST) indicate that the plasma is highly destabilised above z displacements of 0.4m. Previous work investigating the plasma response to vertical plasma perturbations in MAST had found it to be more non-linear than an equivalent conventionally-shaped tokamak. Further investigation into the stability of the plasma at high z displacements is done using a linear vertical stability code. Theoretically it is found that the plasma becomes ideally unstable from z 0:8m, corresponding to a signicant acceleration of the vertical position experimentally.