Hope you are getting by. Unfortunately I have no experience with your problem, but imagine the algorithm that decides about how you pay says no.
It is unacceptable that you are put in such a situation, I have had the pay date change over problem before with my life and it is always a trying time.
May I suggest you contact this page?
There is a lot of active conversation on Mumsnet that may be of benefit to you, and I always find women more willing to talk and be open about situations than men in general.
Maybe give posing the questions a try. Subsequently I do suggest that taking you to court will be more expensive than just working out how to make payments when you can. The shoot in head ask questions later policy is reminiscent of a company making profit off just getting the money in monthly and not actually doing what is best for service users.
Are you able to speak to your ex about the situation? Having a response from her in writing will be highly beneficial if they decided like morons to take things further. Also having your new pay put into an account they cannot touch might be wise.
I hope your situation works out for sure.
All the best