Custom page renderer

Hi all.

I have a custom page renderer in iOS where when it passes through the ViewWillAppear I want to add a button to the nav bar. I think you do it like this as i can't see a way to add it when in the forms project. Code is below, but doesn't seem to add the button at all. Any ideas? Many Thanks, Stuart.

public override void ViewWillAppear (bool animated)
{
base.ViewWillAppear (animated);

var btn = new UIBarButtonItem (UIImage.FromBundle ("threelines")
    , UIBarButtonItemStyle.Plain
    , (sender, args) => {

    });

this.NavigationItem.SetLeftBarButtonItem (btn, true);

}