DD was born in July, so she was a winter baby.

She would wear a singlet, wondersuit then wrapped in 2 blankets and her cot quilt over the top. Our room got far colder than 21 degrees, probably closer to 15 degrees overnight and she seemed quite ok.

If she is waking frequently overnight then she is probably cold. If you can, put a couple of fingers down the front of her clothes and feel her chest. If her chest is warm (but not too hot) then she is probably warm enough. They regulate their temperature (as much as they can) through their head, so you're right not to put a beanie/hat on her. Don't worry if her hands or face feel cool either, as long as her chest is warm then she is warm.