Ballinger ****SPOILERS****

It seems he got away.

Someone please correct me if I'm wrong as I'm no expert on British law. I think what happened was that Tennison had no real evidence of Ballinger's wrongdoing. Therefore, she could not take her complaint to court or to a superior in the police force. Though Tennison might be able to argue that Ballinger showed poor judgement in failing to arrest The Street, Ballinger could just say that there wasn't enough evidence to do so. If anyone found any indication that Ballinger had suspicious contact with The Street, Ballinger could say (and with some truth) that The Street was an informant, so of course the two men were in contact.