Just to follow this one up, I found that the replaceVariableByHTML method would default to the Word document font if I set the strictWordStyles to true. e.g.
$docx->replaceVariableByHTML(Test', 'block',$TestWording, array('isFile' => false, 'parseDivsAsPs' => true, 'downloadImages' => true, 'strictWordStyles' => true)