This is very similar to what I am thinking.
I think BS has seen a photo of William while at grandmas house, and she has mentioned they are coming to visit.
Williams mum said on 60 minutes something about how William could really grab people's attention, and that they would turn their heads to look at him (he is an extremely cute little boy).
Maybe I watch too much Criminal Minds, but even if William isn't to BS's "taste", he may be exactly what other people in the "ring" might want.
Maybe BS has informed the ring of this child and let them know that he is scheduled to go back at some point to fix the washing machine (thus giving him a legitimate reason to go back to the house).
The two cars parked across the road may not have been involved at all (meaning it was solely BS involved) .... OR those cars may have been there as a lookout, to possibly scope out if the parents left the house / kids were playing outside etc. They then could have let BS know the best time to "coincidently" turn up to do the repair.
BS can go to the house and try his luck. If by chance he is spotted, he can simply claim he is there to fix the washing machine. If he's not spotted, then he simply grabs William and flees before anybody even realises.
Of course, if the other cars weren't involved, then he simply took his chance by going there on the Friday, because grandma had already mentioned that's when they would be there.
I'm also thinking that if the other cars were involved, they possibly left just prior to the abduction, but parked somewhere close by, so that BS could probably deliver William straight to them, thus if anybody suspected him at all, he would not have William in his possession.
edit - this theory could also fit in with when BS and MS were being questioned by the media that time and he said something like "I did not hurt him" .... i.e. he personally didn't do anything to William (apart from snatching him)
