Mark I have answered this one here in the past.
If you use the search facillity and type in the key word bivvies you will find all the advice that you will ever need.
However my experience wth the Cypry Excell bivvy was that on the first time out on a weeks trip to France the zips on the doors jammed on one side and gave way on the other and to cap it all it leaked so needless to say I wouldn't recommend them
