diff --git a/app/mailers/merge_completion_mailer.rb b/app/mailers/merge_completion_mailer.rb
index 21128c43e..f2a9f8030 100644
--- a/app/mailers/merge_completion_mailer.rb
+++ b/app/mailers/merge_completion_mailer.rb
@@ -1,16 +1,15 @@
 class MergeCompletionMailer < NotifyMailer
-  MERGE_COMPLETION_TEMPLATE_ID = "xxx".freeze
+  MERGE_COMPLETION_TEMPLATE_ID = "b3b62e72-5ced-4515-8720-08bdc7bac792".freeze
 
-  def send_merge_completion_mail(email, merged_organisation_name, absorbing_organisation_name, merge_date, username)
+  def send_merge_completion_mail(email, merged_organisation_name, absorbing_organisation_name, merge_date)
     send_email(
       email,
       MERGE_COMPLETION_TEMPLATE_ID,
       {
         merged_organisation_name:,
         absorbing_organisation_name:,
-        merge_date:,
+        merge_date: merge_date.to_formatted_s(:govuk_date),
         email:,
-        username:,
       },
     )
   end
diff --git a/app/services/merge/merge_organisations_service.rb b/app/services/merge/merge_organisations_service.rb
index 04f2ea7ba..ead816dd2 100644
--- a/app/services/merge/merge_organisations_service.rb
+++ b/app/services/merge/merge_organisations_service.rb
@@ -155,7 +155,7 @@ private
   def send_success_emails
     @merged_users.each do |organisation_name, users|
       users.each do |user|
-        MergeCompletionMailer.send_merge_completion_mail(user[:email], organisation_name, @absorbing_organisation.name, @merge_date, user[:name]).deliver_later
+        MergeCompletionMailer.send_merge_completion_mail(user[:email], organisation_name, @absorbing_organisation.name, @merge_date).deliver_later
       end
     end
   end
diff --git a/spec/mailers/merge_completion_mailer_spec.rb b/spec/mailers/merge_completion_mailer_spec.rb
index 689932868..0ebe96fcd 100644
--- a/spec/mailers/merge_completion_mailer_spec.rb
+++ b/spec/mailers/merge_completion_mailer_spec.rb
@@ -9,18 +9,17 @@ RSpec.describe MergeCompletionMailer do
   end
 
   describe "#send_merge_completion_mail" do
-    let(:merge_date) { Time.zone.today }
+    let(:merge_date) { Time.zone.local(2023, 1, 1) }
 
     it "sends a merge completion E-mail via notify" do
       expect(notify_client).to receive(:send_email).with(hash_including({ personalisation: hash_including({
         merged_organisation_name: "merged organisation",
         absorbing_organisation_name: "absorbing organisation",
-        merge_date:,
+        merge_date: "1 January 2023",
         email: "user@example.com",
-        username: "user",
       }) }))
 
-      described_class.new.send_merge_completion_mail("user@example.com", "merged organisation", "absorbing organisation", merge_date, "user")
+      described_class.new.send_merge_completion_mail("user@example.com", "merged organisation", "absorbing organisation", merge_date)
     end
   end
 end
diff --git a/spec/services/merge/merge_organisations_service_spec.rb b/spec/services/merge/merge_organisations_service_spec.rb
index d96933e3b..fe6843981 100644
--- a/spec/services/merge/merge_organisations_service_spec.rb
+++ b/spec/services/merge/merge_organisations_service_spec.rb
@@ -838,8 +838,8 @@ RSpec.describe Merge::MergeOrganisationsService do
       end
 
       it "sends a merge completion E-mail to the merged organisation users" do
-        expect(MergeCompletionMailer).to receive(:send_merge_completion_mail).with(merging_organisation_user.email, "fake org", "absorbing org", Time.zone.today, merging_organisation_user.name).once
-        expect(MergeCompletionMailer).to receive(:send_merge_completion_mail).with(merging_organisation.data_protection_officers.first.email, "fake org", "absorbing org", Time.zone.today, merging_organisation.data_protection_officers.first.name).once
+        expect(MergeCompletionMailer).to receive(:send_merge_completion_mail).with(merging_organisation_user.email, "fake org", "absorbing org", Time.zone.today).once
+        expect(MergeCompletionMailer).to receive(:send_merge_completion_mail).with(merging_organisation.data_protection_officers.first.email, "fake org", "absorbing org", Time.zone.today).once
 
         merge_organisations_service.call
       end