Menu Block module and theme hook functions

I'm a complete Drupal newb so this is probably more about my failure to understand theme hook functions, but:

I've got Menu Block running and I'm trying to use it on my Main Menu, in my theme named "wibble". The block is being outputting the following HTML:

<div class="menu-block-wrapper menu-block-1 menu-name-main-menu parent-mlid-0 menu-level-1">
	<ul class="tabs">
		<li class="first leaf active menu-mlid-227"><a href="/" class="active">Home</a></li>
		<li class="last expanded menu-mlid-317"><a href="/about" title="Read all about me">About Me</a>
			<ul class="tabs">
				<li class="first last leaf menu-mlid-385"><a href="/about/sub" title="">More About Me</a></li>
			</ul>
		</li>
	</ul>
</div>

I'm trying to use a hook function to rewrite the output:

function wibble_menu_tree__menu_block($variables) {
  return '<ul class="tabs">' . $variables['tree'] . '</ul>';
}

This is, as the HTML output shows, working fine. However, it'll affect all menu blocks. I want it to only affect the 'Main Menu' block. The wrapper div is coming out with a class "menu-block-1" so I assume my block ID is "1". But if I amend the name of the hook function to be wibble_menu_tree__menu_block__1($variables), no HTML list is output inside the block wrapper.

What am I doing wrong?