I think I must have misunderstood the part about the murder weapon. I thought Frost produced a photo of their first visit which showed a statue which was now missing on this visit and he wants to know why it has gone because it could have been the murder weapon (well he is a detective). Given the amount of blood, surely they would have seen the connection on their first visit?
Did anyone else know immediately on seeing the fellow who had served 20 years that he was innocent?