Near where I live there is an agricultural college and they have cows, sheep, llamas and alpacas and a great pile of manure about 20 feet high. It's free to anyone who wants it, so that's where mine comes from. The good thing is, not many people take advantage of it and so it just builds up. One end has been there for years and so is well rotted, while the other end is this years' offerings and too fresh. I don't know if it's right but I wonder if the really old stuff may have had the goodness washed out of it after standing in the rain for so long - and we do get a fair bit of rain in Wales. I always take mine from half way down.