rickman I don't know if there is any other way. You could of course go to your MP and make him/ her aware of the loophole.
And of course, you could ring up your in-laws and let them know how little he cares about the welfare of his children. But they will put that down to the rantings of a bitter woman, because people always do.
It's a long long haul, but your ex will reap what he's sown when your children are grown up and understand how he has behaved. Meanwhile, I'd say try not to say too much in front of them - I know it's hard - but it damages their sense of self to know that half of their genetic material is a waste of space.
Could you also join a support group like Gingerbread, where you will get sympathy and practical support (like incredibly cheap holidays!)?