Could ps be the neighbourhood busy body who knows everyone on the street and everyone's vehicles, and comings and goings, like he owns the street, did he watch ff arrive that night and by being so nosey did he witness something that morning and that's why gj has been trying to get him to talk?
If he took william when would he do it? Before or straight after his wife left, if w was noticed missing between 10-10.30 ps wife was still there and then the neighbour was knocking on his door just after hs left, so did he hide him in the house with the radio blaring and people banging on his door, then answer it and go straight to fgms house leaving w in his house?
He might be guilty of this and seems suss with his changing stories etc but logistically when would he do it?