In reply to Tall Clare:
Mesh door is an absolute must to stop the biting nasties, and allow you to vent on a warm summers night. I never got a good nights sleep fully sealing up a bivvy in summer temperatures at valley levels.
A pole at the head end helps air flow and stops the fabric touching your face, which always kept me a wake at night.
Bivvy bags (even double poles ones) in prolonged rain are miserable. Cheaper non goretex ones doublely so.............
I bivvied for years but final went for a light tent, more comfortable, space to change out of the rain, somewhere to sit up, cook and read ect.
If you are still interetsed in a bivvy I have a mountain range double hooped bivy for sale, olive green, goretex, about 1.2 Kg's, but there are some pin holes in the base; £50. Cheaper way to try the idea out than spending £250+ on a new one.................
Occasionally, usually high up on a still clear night bivvying is excellent.