Use legend as H1

…seeing as the radios are the only control in the form.

Follows the pattern in
https://github.com/alphagov/notifications-admin/pull/3771/files

app/main/forms.py

@@ -1295,7 +1295,7 @@ class ConfirmPasswordForm(StripWhitespaceForm):
 
 class NewBroadcastForm(StripWhitespaceForm):
     content = GovukRadiosField(
-        "How to do you want to add content to the alert?",
+        'New alert',
         choices=[
             ('freeform', 'Write your own message'),
             ('template', 'Use a template'),

app/templates/views/broadcast/new-broadcast.html

@@ -1,4 +1,4 @@
-{% from "components/page-header.html" import page_header %}
+{% from "components/back-link/macro.njk" import govukBackLink %}
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/form.html" import form_wrapper %}
 
@@ -10,13 +10,19 @@
 
 {% block maincolumn_content %}
 
-  {{ page_header(
+  {{ govukBackLink({ "href": url_for('.broadcast_dashboard', service_id=current_service.id) }) }}
-    'New alert',
-    back_link=url_for('.broadcast_dashboard', service_id=current_service.id),
-  )}}
 
   {% call form_wrapper() %}
-    {{ form.content }}
+    {{ form.content(
+      param_extensions={
+        'fieldset': {
+          'legend': {
+            'isPageHeading': True,
+            'classes': 'govuk-fieldset__legend--l'
+          }
+        }
+      }
+    ) }}
     {{ page_footer('Continue') }}
   {% endcall %}