I'm afraid I've got some bad news.
I've been thinking about the flat and I've decided that it isn't a longterm solution for us:
basically it's a really small place and because you and Sam have got such a great relationship it's more like living with a couple for half the week, than one person for most of the week. it's nothing against you guys personally, it's just that the place is so small that we'll end up tripping over each other. It wouldn't be a problem in a bigger house. I wish I'd thought to ask you about stuff like this before you moved in to save both of us the stress and inconvenience of unstable living arrangements. I know it's only been a short while, but I think it's better to address this now so we can both look for something more permanent.
We can discuss what we'll do from here (how much notice you'll need etc.) when I see you next. One thing you might be interested in is that the identical flat next door has been newly renovated and is coming up for lease soon (I think)?
Again I'm sorry. I know you're busy and that moving is time consuming and unsettling and generally shitty. However I've put a lot of thought into this and I just think it will be the best solution for both of us.