He won't give the kids back please help

Sa

As the title says the father of my children has them during the school holidays because he lives 5 hours away (10 there and back) so we have court orders to say they live with me and he gets them on the holidays. they went over for Christmas and he hasn't returned them and says he isn't going to, they have school tomorrow! I've tried contacting the police because he broke the court order and they advised him to give the kids back but he said no and now there isn't anything anybody can do. what can I do to get them back? :( I'm in the UK if that helps.