Insert element in to sub-menu

I know I can do this with jQuery, but i would much rather do it in php, for obvious reasons.

For my code i have a my main menu as follows:

<ul>
<li>Item 1</li>
<li>Item 2
<ul class="sub">
      <li>Sub Item 1</li>
      <li>Sub Item 2</li>
</ul>
</li>
<li>Item 3</li>

as part of making my site responsive I need to insert the code <li>back</li> as the first child of the sub menu on my main menu.

I’ve seen some of the walker_menu stuff but not sure if that’s where i should be looking?

thanks
dan