Just in case anyone's interested 
William is doing really well. He's moved himself into mine and dh's room and has a den underneath the cabinet. He has more room.
Today, dh found him sitting on the window sill in the sun (South facing), next to OldCat who was fast asleep.
Then, I was sitting quietly just now and I heard a lot of noise upstairs. I figured it was ds1 who is quite loud, so didn't think anything of it. Anyway, the sound continued and sounded like a herd of elephants 
I ventured upstairs and found that ds1 was fast asleep in bed.
It turns out that William had been running around from room to room and had been playing with a toy mouse that had been lying on the landing. I got the feather stick and offered it to him and he started playing with that too.
This is good progress 
I plan on luring him downstairs with Dreamies on each stair on Sunday.
He likes milk and continues to appreciate the aquariums on YouTube
Poor OldCat has had a tooth removed and a full clean. The vet is amazed at her general condition given her age and her bloods came back pretty good. I couldn't get the post op painkiller into her, but she's back eating so I guess she's feeling okay. She's in a good mood these days and they even managed to take blood from her neck vein without using any sedation. You can stroke her without getting shredded too