Was the knife recovered? If so there will be fingerprints which would contradict what he says. If you want to really punish the guy then prove your innocence and have him charged for Perverting the course of Justice. I assume this is in England, I am Scottish and so the laws will be slightly different but you get what I am saying.
From what the description says, there is no case for him battle and by changing his story to an unbelievable lie the 'Taxi Cab Rule' will not apply to his solicitor and he would, if he had any sense drop the case or concede.
Hope it helps,
Fraser
[email protected]