Preventing deletion of nodes with comments

Well I'm going to answer my own question, in case somebody else wants to do this.
Its possible in Drupal 6 by using both the Rules and Content Access modules, which work together. Assuming that the rules are to apply to authorised users who are creating and adding comments to a 'discussion' content type (nothing special about the content type).

Create 2 rules:
1. A rule to grant edit / delete permissions to authorised users who create a new discussion content node.
2. A rule to revoke edit / delete permissions when a new comment is added to that node.

Here are my exported rules, which can be imported using the Rules module's import function:

array (
'rules' =>
array (
'rules_1' =>
array (
'#type' => 'rule',
'#set' => 'event_comment_insert',
'#label' => 'new discussion comment changes node access permission',
'#active' => 1,
'#weight' => '0',
'#categories' =>
array (
),
'#status' => 'custom',
'#conditions' =>
array (
0 =>
array (
'#type' => 'condition',
'#settings' =>
array (
'type' =>
array (
'discussion' => 'discussion',
),
'#argument map' =>
array (
'node' => 'node',
),
),
'#name' => 'rules_condition_content_is_type',
'#info' =>
array (
'label' => 'Commented content is discussion',
'arguments' =>
array (
'node' =>
array (
'type' => 'node',
'label' => 'Content',
),
),
'module' => 'Node',
),
'#weight' => 0,
),
),
'#actions' =>
array (
0 =>
array (
'#type' => 'action',
'#settings' =>
array (
'#argument map' =>
array (
'node' => 'node',
),
'view' =>
array (
),
'view_own' =>
array (
),
'update' =>
array (
0 => 2,
),
'update_own' =>
array (
0 => 2,
),
'delete' =>
array (
0 => 2,
),
'delete_own' =>
array (
0 => 2,
),
),
'#name' => 'content_access_action_revoke_node_permissions',
'#info' =>
array (
'label' => 'Revoke content permissions by role',
'arguments' =>
array (
'node' =>
array (
'type' => 'node',
'label' => 'Content',
),
),
'module' => 'Content access',
),
'#weight' => 0,
),
),
'#version' => 6003,
),
'rules_2' =>
array (
'#type' => 'rule',
'#set' => 'event_node_insert',
'#label' => 'new discussion grants edit and delete permissions',
'#active' => 1,
'#weight' => '0',
'#categories' =>
array (
),
'#status' => 'custom',
'#conditions' =>
array (
1 =>
array (
'#weight' => 0,
'#type' => 'condition',
'#settings' =>
array (
'type' =>
array (
'discussion' => 'discussion',
),
'#argument map' =>
array (
'node' => 'node',
),
),
'#name' => 'rules_condition_content_is_type',
'#info' =>
array (
'label' => 'Created content is discussion',
'arguments' =>
array (
'node' =>
array (
'type' => 'node',
'label' => 'Content',
),
),
'module' => 'Node',
),
),
0 =>
array (
'#weight' => 1,
'#info' =>
array (
'label' => 'Created content is new',
'arguments' =>
array (
'node' =>
array (
'type' => 'node',
'label' => 'Content',
),
),
'module' => 'Node',
),
'#name' => 'rules_condition_content_is_new',
'#settings' =>
array (
'#argument map' =>
array (
'node' => 'node',
),
),
'#type' => 'condition',
),
),
'#actions' =>
array (
0 =>
array (
'#weight' => 0,
'#info' =>
array (
'label' => 'Grant content permissions by role',
'arguments' =>
array (
'node' =>
array (
'type' => 'node',
'label' => 'Content',
),
),
'module' => 'Content access',
),
'#name' => 'content_access_action_grant_node_permissions',
'#settings' =>
array (
'#argument map' =>
array (
'node' => 'node',
),
'view' =>
array (
0 => 1,
1 => 2,
),
'view_own' =>
array (
0 => 1,
1 => 2,
),
'update' =>
array (
),
'update_own' =>
array (
0 => 2,
),
'delete' =>
array (
),
'delete_own' =>
array (
0 => 2,
),
),
'#type' => 'action',
),
),
'#version' => 6003,
),
),
)