Yup - there's been a sharp uptick recently in the amount of fresh stuff left around. I think one or two people (or, more specifically, their dogs) make a huge and visible difference on this. Wondering whether it's dogs for Christmas or just a failure to be bothered to pick up in the cold weather (or walk far enough to be in a park).
There's a lengthy discussion about it on a previous thread here.
Contact the council through the LoveLewisham website (google it) and they're generally pretty responsive, though the thread above has numerous other options.
[Admin, might be worth merging this thread into the existing one...]