Add some context to the list of reports

We reckon people need some context/expectation setting about what the
date of the report is.

app/templates/views/returned-letter-summary.html

@@ -1,5 +1,6 @@
 {% from "components/table.html" import list_table, row_heading %}
 {% from "components/message-count-label.html" import message_count_label %}
+{% from "components/page-header.html" import page_header %}
 {% extends "withnav_template.html" %}
 
 {% block service_page_title %}
@@ -7,9 +8,20 @@
 {% endblock %}
 
 {% block maincolumn_content %}
-<h1 class="heading-large">
-    Returned letters
-  </h1>
+<div class="grid-row">
+  <div class="column-five-sixths">
+    {{ page_header(
+      'Returned letters',
+      back_link=url_for('main.service_dashboard', service_id=current_service.id)
+    ) }}
+    <p class="govuk-body">
+      Reports are published once a month.
+    </p>
+    <p class="govuk-body">
+      You’ll only get a report if one or more of your letters is returned.
+    </p>
+  </div>
+</div>
 <div class="dashboard-table">
     {% call(item, row_number) list_table(
         data,