Clusters in our sample in diagrams of useful XMM-Newton exposure time versus redshift. Filled symbols are clusters with one or more detected substructures, and open symbols clusters without any detected substructures. Solid lines are susbstructure detection limits below which it is not possible to detect a substructure. Upper panel: red symbols are clusters brighter than 5.0 × 10⁴⁴ erg/s with detected substructures fainter than 2.5 × 10⁴⁴ erg/s and more distant than 7 pixels from the cluster centre (or no detected substructures). Blue symbols are clusters brighter than 5.0 × 10⁴⁴ erg/s with detected substructures brighter than 2.5 × 10⁴⁴ erg/s and more distant than 7 pixels from the cluster centre. Middle panel: red symbols are clusters brighter than 5.0 × 10⁴⁴ erg/s with detected substructures fainter than 2.5 × 10⁴⁴ erg/s and less distant than 7 pixels from the cluster centre (or no detected substructures). Blue symbols are clusters brighter than 5.0 × 10⁴⁴ erg/s with detected substructures brighter than 2.5 × 10⁴⁴ erg/s and less distant than 7 pixels from the cluster centre. Lower panel: red symbols are clusters fainter than 5.0 × 10⁴⁴ erg/s with detected substructures fainter than 0.4 × 10⁴⁴ erg/s and less distant than 7 pixels from the cluster centre (or no detected substructures).