Good morning Iain,
This is actually just a design decision, Sensible TableView gives you the choice whether to edit the task or not. To achieve what you're looking for, go to PeopleViewController.m under the PersonEntity class definition and change the following line:
((SCArrayOfObjectsAttributes *)tasksPropertyDef.attributes).allowEditingItems = NO;
To the following:
((SCArrayOfObjectsAttributes *)tasksPropertyDef.attributes).allowEditingItems = YES;
(or just remove the line all together as this is the default behavior)