General theory running through mine and I'm sure many other heads-
*the ring involves people within the community, including at one point BS. Probably still does.
*His flat above the store is used as a pitstop for the children they grab, the kids are then moved on to another location
*BS knows the person who has William because he grabbed him for him/her, held him overnight at the flat and then moved him on elsewhere.
*BS's current charges are possibly because of the publicity he got and he's being used to grab the other mongrels.
As I said, just random linking of what we know with other things that have been reported.