Use the 404 page

public/404.html

@@ -2,8 +2,16 @@
   <main class="govuk-main-wrapper govuk-main-wrapper--l" id="main-content" role="main">
     <div class="govuk-grid-row">
       <div class="govuk-grid-column-two-thirds">
-        <h1 class="govuk-heading-l">Sorry, there is a problem with the service</h1>
-        <p class="govuk-body">Try again later.</p>
+        <h1 class="govuk-heading-l">Page not found</h1>
+        <p class="govuk-body">
+          If you typed the web address, check it is correct.
+        </p>
+        <p class="govuk-body">
+          If you pasted the web address, check you copied the entire address.
+        </p>
+        <p class="govuk-body">
+          If the web address is correct or you selected a link or button, <a href="#" class="govuk-link">contact the Tax Credits Helpline</a> if you need to speak to someone about your tax credits.
+        </p>
       </div>
     </div>
   </main>