  We present a non-linear analysis of perturbations around cosmological
solutions in Generalised Massive gravity. This Lorentz invariant theory is an
extension of de Rham, Gabadadze, Tolley massive gravity that propagates $5$
degrees of freedom while allowing stable open FLRW cosmologies. For a minimal
model that supports a self-accelerating background, we study the dynamics of
non-linear perturbations. We find that the equation for the scalar graviton is
distinct from the analogues in Horndeski and DHOST theories. We numerically
solve the equation to find a new type of nonlinear solution for the scalar
mode, and confirm the presence of a Vainshtein screening mechanism. We show
that the PPN parameter approaches its GR value at solar system scales and
satisfies the current bounds.
