Evidence of a complex failure mechanism which involved both a distinct planar sliding surface, in addition to block-flexural toppling has contributed to the instability at a UK slate quarry. The failure was investigated using the Discrete Element Method (DEM) and the Finite Element Method (FEM). The DEM in Universal Discrete Element Code (UDEC) has been found to be better in simulating the structurally controlled failure in comparison to FEM in Phase2. The UDEC model has successfully provided further insight into the possible mechanisms associated with the observed deformations. The modelling has confirmed the influence of the water table and the presence of a tension crack which contributes to the instability.