If anyone's interested in how this turned out, the black polish girl who started out with the silkies just stopped sitting and went back to the flock. The other (black and white) polish is still clucky, but she's now pancaked on her own in another nest box.
The 2 silkies hatched the babies, BUT their "mum" (the right hand chook in my avatar, who hatched them) has stepped in and she's now in there with them. I'm quite happy about that because she is my "top chook" and I've seen her pecking at other chooks to stay away from the chicks. I was worried about the babies getting attacked, but with Flossie in there, I'm not so worried.