I decided against a heater in the end I got some base layers and thermal work trousers with my old fishing suit for extreme weather they got too hot last time though so I stripped off 🔥😁
Get a nice hot water bottle or two for the sleeping bag that seemed to work well last time, one for the feet and one for your back lovely, or stagger the time you boil the kettle for them that way you will stay warmer for longer