Getting the URI of an Entries field in a Structure with no URLs
So this is what worked for me. It's a bootstrap drop-down menu system using the nav tag from the structure 'menu' with custom fields as described above

&lt;ul class="nav navbar-nav navbar-right"&gt;
{% set menu = craft.entries.section('menu').hidePage('&lt; 1') %}
{% set navlink = "" %}

{% nav entry in menu %}
    {% if entry.entryLink.first|length %}
    {% set navlink = entry.entryLink.first.uri %}
    {% else %}
    {% set navlink = entry.customMenuUrl %}
    {% endif %}

    {% if entry.level == 1 and entry.children|length %}
        &lt;li class="dropdown"&gt;
            &lt;a  href="{{ siteUrl }}{{ navlink}}"  role="button" data-hover="dropdown" data-length="{{ entry.title|length }}" aria-haspopup="true" aria-expanded="false"&gt;{{ entry.title }}&lt;span class="caret"&gt;&lt;/span&gt;&lt;/a&gt;
            &lt;!-- get children --&gt;
            &lt;ul class="dropdown-menu" &gt;
                &lt;!-- get children --&gt;
                {% for child in entry.getDescendants().hidePage('&lt; 1') %}
                {% if child.entryLink.first|length %}

                &lt;li&gt; &lt;a  href="{{ siteUrl }}{{  child.entryLink.first.uri }}" class="{{ macros.isActiveChildClass(child.slug) | trim }} level{{ child.level }}"&gt;{{ child.entryLink.first.title }}&lt;/a&gt;&lt;/li&gt;
                {% endif %}
                {% endfor %}
            &lt;/ul&gt;
        &lt;/li&gt;
    {% elseif entry.level == 1 and not entry.hasDescendants() %}
     &lt;li&gt;&lt;a href="{{ siteUrl }}{{ navlink }}" class="{{ macros.isParentMenuActive(entry.slug) |trim }} level{{ entry.level }}"&gt;{{ entry.title }}&lt;/a&gt;&lt;/li&gt;
    {% endif %}


{% endnav %}




This has managed to produce a bootstrap drop down menu from the nav tag on a structure section with two fields: customMenuUrl (plain text) and entryLink (Entries field)