My neighbour had her loft insulation increased under that scheme. Her loft apparently wasn't insulated to the current standard although, like mine it was always very cold up there in winter, with insulation up to the level of the rafters. She now has literally pools of condensation in her loft due apparently to the new insulation affecting the airflow. The people who did the job are saying its nothing to do with the new insulation but "as a gesture of good faith" are coming back "to see what they can do".
free isn't always a good idea.