thanks for the reponse, I switched over to using:
$html = '<span style="font-family: Arial;font-size:11pt;background-color: yellow;">9</span>';
$docx->replaceTemplateVariableByHTML('FIELD_BK', 'inline', $html);
but I still don't get a highlighted background.
I tried setting the colour with a #ffff00 with the same result. Any other suggestions would be greatly welcomed.