The ending makes it clear that Mary Maloney manages to commit the perfect murder by getting the police officers who visit the property to investigate themselves to dispose of the murder weapon. If you look at the ending, you see that the police officers actually eat the leg of lamb with which Mary killed her husband, and what is even more darkly ironic, they talk about the murder weapon whilst they eat it:
One of them belched.
“Personally, I think it’s right here on the premises.”
“Probably right under our very noses. What you think, Jack?”
And in the other room, Mary Maloney began to giggle.
This shows that Mary through her cunning has managed to kill her husband and use her knowledge as a policeman's wife to recognise how important it is to get rid of the murder weapon so it cannot be found or linked to her in any way. She has found the perfect way of doing this through cooking the leg of lamb and serving it to the officers. Her innocence and future, and that of her baby, is assured.