Thanks. I've changed the code as you suggest and reactivated the plugin and the menu link still works.
I'd still like to know why the appearance changes at smaller resolutions. I could probably work it out if I could see the style sheet the developer is using but in the Editor styles.css has nothing visible but comments.
How do I view the actual style sheet? All I see is:
Theme Name: Neutro
Theme URI: http://seotemplates.net/blog/theme/neutro-wordpress-theme/
Author: Septian Ahmad Fujianto
Author URI: http://seotemplates.net/
Description: A responsive modern design theme geared to unleash your website content potential. Built based upon HTML5, CSS3 and the rock solid Hybrid Core framework, ensuring the best design and backend code. Support WordPress post formats, with specially tailored design for every supported post formats. Built in touch ready featured content slider with configurable content options, you can easily show your best content to audiences. With Theme customizer and custom css, you can easily customize the theme look and feel to match your desire. The perfect starting point theme for any kind of website you desired.
License: GNU General Public License
License URI: http://www.gnu.org/licenses/gpl.html
Tags: gray, white, green, blue, two-columns, three-columns, right-sidebar, flexible-width, custom-background, custom-colors, custom-menu, featured-images, post-formats, sticky-post, theme-options, threaded-comments, translation-ready
Text Domain: Neutro
I emailed the developer last week about this but have had no reply so I assume I have to work it out for myself. I could if I could see the stylesheet.