Keeping Warm in Winter?

Hi. I am planning on braving the winter, and doing some decent winter
sessions this year. Any tips?

I have my eyes on a JRC 5-Season sleeping bag, and will wrap up in plenty of
jumpers, etc.

What are bivvy heaters like? Are they worthwhile?

Also, someone told me that he has seen rods go in during the winter where
the line had froze to the eyes of the rod. Is there any way of preventing
this?

Cheers.