Why is SplineComponent not visible?

Any ideas why

ASplineActor* SplineActor = Cast<ASplineActor>(TheWorld->SpawnActor(ASplineActor::StaticClass()));

(ASplineActor being a custom actor with a spline component)

is not showing up in game or even if I add points to an instance I place within the editor?
I also use SetHiddenInGame(false) on the spline component itself.

For future reference if anybody else has this issue - you need to call MarkRenderStateDirty() on the spline component after adding points to it. From what I can see this is necessary in order to push the updates across to the SceneProxy object that represents the spline component.
You will also need to toggle display of splines by doing the following (with the appropriate null checks):