Rendering a template from a controller - unable to find template
Yes I was being dumb....
I wasn't returning the rendered template. Controller should be
public function actionLoadHolidays()
    {
        $request = Craft::$app-&gt;request;
        $destination = $request-&gt;post('destination');
        
        $holidays = [];
        
        $oldMode = Craft::$app-&gt;view-&gt;getTemplateMode();
        Craft::$app-&gt;getView()-&gt;setTemplateMode(View::TEMPLATE_MODE_SITE);
    
        return $this-&gt;renderTemplate('how-to-book/holidays-for-destination-options.twig', array(
            'destination' =&gt; $destination,
            'holidays' =&gt; $holidays
        ));

    }