They're at about the right age to start weaning now - in fact Mom's 'withdrawing slightly' behaviour is just about the clearest sign that she reckons they're ready and is starting/wanting to start the process. You can help by putting out some food, water and a tray. Here's some guidance - it's the ASPCA but still pretty good.
RE homes, I know we will be keeping one girl and a friend wants the other girl, so 3 black boys need homes. I've not tried to find them any yet, not sure how to?
The litter tray wont fit in the box they are in so do I give them free range of the kitchen now? and buy a cat bed? Thats a bit worrying as alot of small kids running about and I wouldnt want them to get hurt? They are stil super tiny.
I had a quick look see and Whiskas Milk shouldn't be fed to kittens under 6 weeks old anyway - so hold off on that. I'd be tempted just to give them water anyway on the basis that they'll still be getting milk from Mom. (Even if they start eating food, they'll still be suckling from her until 8 weeks or so and then they should be getting the nutrients they need from their food.)
On the tray/box situation. Lordy. I've always had enough room to give them separate space in some way - did you say you had a garden room? (Maybe I'm misremembering.) It could be that a move in there would be helpful because very shortly they're going to be into everything and DCs running around with kittens in a kitchen is - if not a recipe for disaster, full of potential for problems. I'd try a move because you should be leaving a tray out now (in my view) for Mom to start training them.
Start spreading the word even wider among friends and family/acquaintances/neighbours - people that you like and can trust - that you'll be having three black boys needing homes shortly. Good luck.
PS - you don't need a 'cat bed'. Something soft and draft free should be fine. I'd be inclined to keep their current box and maybe cut down one side a little if it's cardboard so that they can get in easily. A big towel scrunched up on the bottom will be comfy for them and can be hot washed when they leave it.