diff --git a/code/admin/SubsiteAdmin.php b/code/admin/SubsiteAdmin.php
index 77b8841e..e921031f 100644
--- a/code/admin/SubsiteAdmin.php
+++ b/code/admin/SubsiteAdmin.php
@@ -15,14 +15,13 @@
  */
 class SubsiteAdmin extends ModelAdmin
 {
-
     private static $managed_models = [Subsite::class];
 
     private static $url_segment = 'subsites';
 
     private static $menu_title = 'Subsites';
 
-    private static $menu_icon = 'subsites/images/subsites.png';
+    private static $menu_icon_class = 'font-icon-tree';
 
     public $showImportForm = false;
 
@@ -40,5 +39,4 @@ public function getEditForm($id = null, $fields = null)
 
         return $form;
     }
-
 }
diff --git a/code/extensions/LeftAndMainSubsites.php b/code/extensions/LeftAndMainSubsites.php
index 51887bf5..78d315d1 100644
--- a/code/extensions/LeftAndMainSubsites.php
+++ b/code/extensions/LeftAndMainSubsites.php
@@ -257,13 +257,15 @@ public function onBeforeInit()
         // We are accessing the CMS, so we need to let Subsites know we will be using the session.
         Subsite::$use_session_subsiteid = true;
 
+        $session = Controller::curr()->getRequest()->getSession();
+
         // FIRST, check if we need to change subsites due to the URL.
 
         // Catch forced subsite changes that need to cause CMS reloads.
         if (isset($_GET['SubsiteID'])) {
             // Clear current page when subsite changes (or is set for the first time)
-            if (!Session::get('SubsiteID') || $_GET['SubsiteID'] != Session::get('SubsiteID')) {
-                Session::clear("{$this->owner->class}.currentPage");
+            if (!$session->get('SubsiteID') || $_GET['SubsiteID'] != $session->get('SubsiteID')) {
+                $session->clear("{$this->owner->class}.currentPage");
             }
 
             // Update current subsite in session
diff --git a/code/model/Subsite.php b/code/model/Subsite.php
index 18c3f7c1..63fcc2b5 100644
--- a/code/model/Subsite.php
+++ b/code/model/Subsite.php
@@ -4,6 +4,7 @@
 
 use SilverStripe\Admin\CMSMenu;
 use SilverStripe\CMS\Model\SiteTree;
+use SilverStripe\Control\Controller;
 use SilverStripe\Control\Director;
 use SilverStripe\Control\Session;
 use SilverStripe\Core\Convert;
@@ -154,7 +155,7 @@ public static function currentSubsiteID()
         if (isset($_GET['SubsiteID'])) {
             $id = (int)$_GET['SubsiteID'];
         } elseif (Subsite::$use_session_subsiteid) {
-            $id = Session::get('SubsiteID');
+            $id = Controller::curr()->getRequest()->getSession()->get('SubsiteID');
         }
 
         if ($id === null) {
@@ -184,7 +185,7 @@ public static function changeSubsite($subsite)
             $subsiteID = $subsite;
         }
 
-        Session::set('SubsiteID', (int)$subsiteID);
+        Controller::curr()->getRequest()->getSession()->set('SubsiteID', (int)$subsiteID);
 
         // Set locale
         if (is_object($subsite) && $subsite->Language !== '') {
@@ -677,7 +678,7 @@ public function getCMSFields()
                 new Tab(
                     'Configuration',
                     _t('Subsite.TabTitleConfig', 'Configuration'),
-                    new HeaderField('ConfigForSubsiteHeaderField', $this->getClassName() . ' configuration'),
+                    new HeaderField('ConfigForSubsiteHeaderField', 'Subsite Configuration'),
                     new TextField('Title', $this->fieldLabel('Title'), $this->Title),
 
                     new HeaderField(
@@ -783,7 +784,7 @@ public function validate()
     {
         $result = parent::validate();
         if (!$this->Title) {
-            $result->error(_t('Subsite.ValidateTitle', 'Please add a "Title"'));
+            $result->addError(_t('Subsite.ValidateTitle', 'Please add a "Title"'));
         }
         return $result;
     }
@@ -855,14 +856,6 @@ public function absoluteBaseURL()
         return Director::absoluteBaseURL();
     }
 
-    /**
-     * @todo getClassName is redundant, already stored as a database field?
-     */
-    public function getClassName()
-    {
-        return $this->class;
-    }
-
     /**
      * Javascript admin action to duplicate this subsite
      *
diff --git a/templates/SilverStripe/Admin/LeftAndMain_Menu.ss b/templates/SilverStripe/Admin/LeftAndMain_Menu.ss
index d6f32668..86ad94ed 100644
--- a/templates/SilverStripe/Admin/LeftAndMain_Menu.ss
+++ b/templates/SilverStripe/Admin/LeftAndMain_Menu.ss
@@ -1,5 +1,7 @@
-<div class="cms-menu cms-panel cms-panel-layout west" id="cms-menu" data-layout-type="border">
-    <div class="cms-logo-header north">
+<div class="cms-mobile-menu-toggle-wrapper"></div>
+
+<div class="fill-height cms-menu cms-panel cms-panel-layout" id="cms-menu" data-layout-type="border" aria-expanded="false">
+    <div class="cms-menu__header">
         <% include SilverStripe\\Admin\\LeftAndMain_MenuLogo %>
         <% include SilverStripe\\Admin\\LeftAndMain_MenuStatus %>
 
@@ -8,7 +10,7 @@
         <% end_if %>
     </div>
 
-    <div class="panel panel--scrollable panel--triple-toolbar cms-panel-content">
+    <div class="flexbox-area-grow panel--scrollable panel--triple-toolbar cms-panel-content">
         <% include SilverStripe\\Admin\\LeftAndMain_MenuList %>
     </div>
 
@@ -16,3 +18,5 @@
         <% include SilverStripe\\Admin\\LeftAndMain_MenuToggle %>
     </div>
 </div>
+
+<button class="fill-height fill-width cms-menu-mobile-overlay" aria-controls="cms-menu" aria-expanded="false"></button>