Hi Martin and welcome.
Insurance companies often dictate a specific time frame for a hire car. As the car went to an 'approved' repairer, and was not repaired to an acceptable standard, then I think the insurance company should extend your hire car period, especially if they recommended this place.
If they do want the car back though I don't think you'll have much of a leg to stand on as I would lay money on this bieng covered in the t&c's.
Hope you get it sorted soon mate.