Try and get a closure report from social services - it might be useful evidence. You'll need to enforce the order if she's witholding your child, but it would really help to have something in writing from social services to say case was closed. Sometimes they tell you there isn't anything and then there's not much you can do. You could apply for a subject access report, but that could take some time.
Anyway, as the case was closed you need to enforce the order on a C79 - you'll get a hearing quicker than if you do a C100. If you have to do this more than once then each time a black mark will be against your ex. Hopefully an enforcement will deter her from breaching but if not, you could go back to vary for more time or transfer of residence if she keeps doing these things.
Really you should enforce by the third breach. Be prepared for her wapping in an application to vary on the back of your enforcement application, but if she does, you could ask the court to dismiss it as vexatious.