Although it is Nutter when using it in stress you should pronunce it 'Nuttah'
(Yelling across the hallway)"Jack! I think you're a Nuttah'!"
Also probably the most common mistake made by outsiders when trying to reproduce a British accent. For the record, it's either "he's a nutter" or "he's nuts". Saying "he's nutters" is a good way to make British people dislike you.
Bad: "Are you completely nutters?"