Often regarded the preserve of climbers perched on inaccessible rocky ledges or soldiers hidden in the bushes, in the right conditions the bivvy (bivi, bivvi or bivouac) bag can add a whole new level of enjoyment to an overnight camp. They’re basically a waterproof cover for a sleeping bag, ideally made from a breathable fabric to reduce condensation build-up within, and sometimes fitted with a zip to make access easier. Combined with a sleeping bag and a ground mat (placed within the bivvy bag or left outside) the small footprint makes it possible to sleep in places where a tent…