populate_list - Relationship

Hi all.

I'm looking to simply use populate_list to pull a relationship from Leads into Opportunities. I've created what I thought was the correct simple file of:

$dictionary["Lead"]["fields"]["opportunity"]['populate_list'] = [
		'contacts_leads_1_name' => 'contacts_opportunities_1_name',

However this has failed to work, I've put this file here - custom/Extension/modules/Leads/Ext/Vardefs

Simply it should pull the Contact through to the Opp in the custom relationship. Have I missed anything obvious?