The present invention relates to compounds of formula I wherein R1, R4, Ar, W, X and Z are all as defined herein. The compounds of the present invention are known to inhibit the spindle checkpoint function of Monospindle 1 (Mps1—also known as TTK) kinases either directly or indirectly via interaction with the Mps1 kinase itself. In particular, the present invention relates to the use of these compounds as therapeutic agents for the treatment and/or prevention of proliferative diseases, such as cancer. The present invention also relates to processes for the preparation of these compounds, and to pharmaceutical compositions comprising them.