diff --git a/README.md b/README.md
index 51dc0f9..bfdf3dd 100644
--- a/README.md
+++ b/README.md
@@ -27,9 +27,31 @@ sourced as environment variables to be retrieved at runtime . See the
 helper-script `bin/ansible-with-atl-env` and the corresponding
 `groups_vars/aws_node_local.yml` var-file.
 
-### Maintenance playbooks
+#### Overriding parameters
 
-(None currently; TBW)
+If you want to customise the playbook behaviour the simplest method is to fork
+this repository and add your own. However, for some one-off tasks you can also
+override the default and calculated settings with special values. To do this,
+provide command-line overrides to
+[ansible-playbook](https://docs.ansible.com/ansible/latest/cli/ansible-playbook.html).
+
+The most likely use-case for this is to download a custom product distribution
+for testing (for example, a pre-release version of Jira). If you are running
+ansible-playbook directly, the command for this would look like the following:
+
+    ansible-playbook \
+        -e atl_product_download_url=http://s3.amazon.com/atlassian/jira-9.0.0-PRE-TEST.tar.gz \
+        -e atl_use_system_jdk=true \
+        -e atl_download_format=tarball \
+        \
+        -i inv/aws_node_local aws_jira_dc_node.yml
+
+You can also do this on a CloudFormation template where the stack details are in
+/etc/atl.  On such templates, the variable `ATL_ANSIBLE_EXTRA_PARAMS` is added
+to the `ansible-playbook` parameters in `bin/ansible-with-alt-env`. In this case
+you need to set it to:
+
+    ATL_ANSIBLE_EXTRA_PARAMS="-e atl_product_download_url=http://s3.amazon.com/atlassian/jira-9.0.0-PRE-TEST.tar.gz -e atl_use_system_jdk=true -e atl_download_format=tarball"
 
 ## Development
 
diff --git a/bin/ansible-with-atl-env b/bin/ansible-with-atl-env
index 14a9938..55b6e8e 100755
--- a/bin/ansible-with-atl-env
+++ b/bin/ansible-with-atl-env
@@ -15,4 +15,8 @@ set +a
 
 # Use Ansible from virtualenv if provided
 pipenv run \
-       ansible-playbook -v -i $INV $PLAYBOOK 2>&1 | tee --append $LOG_FILE
+    ansible-playbook -v \
+        $ATL_ANSIBLE_EXTRA_PARAMS \
+        -i $INV \
+        $PLAYBOOK \
+        2>&1 | tee --append $LOG_FILE