CakePHP Questions

I think that I may be misunderstanding how View Blocks in 2.1 are supposed to work.

I am trying to wrap the site navigation (in the layout) in a view block, so that I can hide it from other views (registration form in this case).

I've read through the documentation and here is what I expected to work:

// In the layout

<nav class="site-navigation">
    <div class="container">
        <?php $this->start('site-navigation'); ?>
            // The HTML here for the navigation menu ...
        <?php $this->end(); ?>
    </div>
</nav>

I expected that this would take my navigation HTML, stick it in the 'site-navigation' block, and display it.

The first problem was that it didn't display so just after the call to $this->end() I added

 <?php echo $this->fetch('site-navigation'); ?> 

to display the menu.

Then at the top of my registration form view I have

 <?php $this->assign('site-navigation', ''); ?> 

to hide the 'site-navigation' content on the registration page.

Unfortunately, this didn't work, the navigation menu is still displayed.

Can anyone point me in the right direction? Are view blocks suitable for my task or have I misunderstood their purpose?

Thanks in advance.