I have been fishing winter nights since the mid-80s and I am so glad that the tackle industry started to revolutionize, it was very bleak and cold in those days. My feet still feel the cold a lot and I am sure it down to those days.
I have put together a list of ways to improve your time on the bank in your bivvy. I am still working on keeping warm outside the bivvy! I am sure that’s also down to, to many winter piking days in wellies and inadequate clothing.
1. Always use a ground sheet and even in the summer, damp rises.
2. Use an old piece of carpet or a thermal blanket under you bedchair
3. With modern bivvy/brolly systems, there is a skirt then you can tuck under the groundsheet.
4. Use items of luggage as an insulation around this area of the Bivvy e.g rod bag, rod sleeves bags etc, this will help keep the draft out and even more insulation on the ground.
5. Also with modern brolly/bivvy use the door letterbox style and don’t open from the bottom up. Tuck the flap under the groundsheet and make sure the zips are full down.
6. Use a bivvy mat or any old door mat.
This all helps to keep you warm through those hard winter days.
Hope this helps