Special Guardian csa payments

Ok so about 5 years ago my ex decided it would be a good idea to disappear with my son. I couldn't find them for around 12 months until I got a call from the social services saying that my son and his half sister had been removed from my ex's care and were about to be put in the care of their "grandmother" (not blood related). I had to go along with it all as it was already set in stone. I have been paying maintenance ever since, however I have heard that I shouldn't be paying anything as my boy is in the care of a special guardian and his mother isn't paying anything. I find this odd but I can't find anyone who will give me a straight answer. Any help will be much appreciated.