I believe that he threatened her and was verbally abusive, and that this was witnessed and possibly recorded by someone else, which is why he's been charged. It was then cooked up by Katie and perhaps Kirsty when they realised that it'd be in the papers to imply that he'd punched her, to not only get revenge on him (we've seen how spiteful and vengeful she can be to her previous partners) and of course, for Katie's main motivation in life; money. Everything that followed, eg. Kieran being called to pick up the kids etc, was all part of the lie.
I had never considered it until tonight, perhaps as I am too naive to believe that even she could stoop low enough to make a false claim of domestic violence, but it's all clicking into place for me now. Like others have said, it didn't look like a fresh bruise and coincidentally tied in to her recently having surgery, which could have caused the bruising. At the time I thought it was strange that she went to the papers the next day; from my experience as a social worker and of experiencing domestic violence myself, victims of domestic violence go through a range of emotions including denial, fear, and shame - many find it difficult to enough to show their injuries to their families and the police, let alone the national newspapers straight after for the whole country to see. If it was a bullshit story to make money from the papers then it would explain her reasons for this. The fact that the police escorted Katie & Carl a week or two later to the Mucky Mansion while they were both in HIS car also seemed strange.. the only reason I can think of is that by this point they'd reunited and spun the police the story that it was all down to Kirstie, and were expecting to see her at MM? Or they convinced police that Kirstie had done sort of damage to the house or something of that nature.
Either way, if it is indeed a lie, Carl is such a numpty for putting up with it and staying with her. He says nobody can control Katie.. if that's the case and he can't control or coerce her, then neither could Kirsty. Even if it was Kirstie's brainwave to begin with, it was Katie who executed it. This might be the lowest thing she's ever done.