I think that, if you want the RSPCA to turn up at the door then to let the RSPCA do their own thing on this.
It's criminal what this woman is doing, I agree, and very sad that we don't have the authority to act legally. If you have a phone number, as I am sure Susan will have in order to be able to get the kitten, then that should be given to the RSPCA and dealt with by them.