Of course be 'ecologically sensitive' as well. How important is the wood to the environment of the location in which you find it? In a forest / moor environment it could be highly important to the eco-system and 'no go'. In a skip it's unlikely to be so. In the extreme you could go out and harvest peat (decayed woody stuff) off the fells - but I doubt too many of us would advocate that unless it was sustainable. That's part of the answer - if what you are doing is ecologically sound and sustainable, amd doesn't legitimately upset a ladowner, and is not commercially profit making, and, and, and, then its PROBABLY OK.