.banner {
  width: 100%;
  Left: 0px;
}

<header class="banner">
  <div class="banner-wrapper">
            <div class="banner-title">
                                            {{ portal | logo }}
                                </div>
                                <nav class="banner-nav">
                                            {{ portal | welcome_navigation }}
                                </nav>
                    </div>
   <nav class="nav-menu" id="header-tabs">
      <ul>
            <li class="nav-menu-title"><a href="https://www.TEST.co.uk" class="nav-links">Back to Main Site</a></li>
        <li class="nav-contact">| Questions? Get in touch: <li class="nav-number">12345568693</li>
     </ul>
    </nav>
        </header>