Pour the water and broth into a large pot and add the venison chunks. Bring this to a bare simmer, just to bubbling. You will notice lots of scum collecting on the surface of the soup. Skim it as best you can. I let the venison gently simmer for 20 minutes, then fish out the venison pieces and put them in a bowl. I then pour the broth through a paper towel set in a sieve over another pot or large bowl. This strains out all the scum. If you skip this step your soup will be cloudy, but still perfectly edible. I just like clear soup.
Add salt to the broth, and return it and the venison to the heat -- only this time do not let it even simmer. Cover the pot and set it on low heat. You are aiming for about 160 to 175°F. Cook the meat this way until tender, which will take 2-3 hours for an old deer, or 90 minutes for lamb.
Add all the vegetables and the barley, cover and cook for another hour or so, or until the barley is tender.
Stir in the chopped nettles and add some black pepper, and just heat this through, about 5 minutes. Serve at once with a dark ale or red wine.