Catch Box HTML validation failure

I had a try at getting my website http://rgneighbours.net to validate using the W3C HTML validator, and consistently get an error that I don't think I can correct without modifying the theme. I've also set up a test site with a simple page and no plugins using the Catch Box theme, at http://rgneighbours.elementfx.com/catch-box-test/ - and it does exactly the same. The cause of the error seems to be that <div class="menu-access-wrap clearfix"> doesn't have a closing </div> tag - when I insert the tag manually before the line </header><!-- #branding --> (I think that's the right place) and paste the code into the validator it's OK. Perhaps the necessary </div> tag could be added to the theme at a convenient time? Many thanks! David.