Thanks for replies. Big is good as it's a long garden and has a tall, ugly block of flats behind. I haven't broached the idea with neighbours yet; I obviously need their ok as the tree is the only interesting and obvious thing in our garden (two of the flats have tenants and the third flat upstairs may come up for sale I suspect so they might not like an established tree going.)
I also wonder whether need to check with the council but then a tree surgeon might tell me!
Also is it unrealistic to get a new tree to grow big in a big pot then swap?!! I'm totally new to gardening but just hoping don't need to live with a big empty looking space for ages. Planting it in the other corner wouldn't work to block out the flats so it needs to be where the other is!
If you are in London, it is highly likely to have a tree preservation order on it. Even if it doesn't, you will need permission. Your tree surgeon can sort that out.
Trees do not like pots. They retard their growth.
It's also worth noting that whilst Acacia pravissima are very beautiful, they do not grow to anything like the size of a sycamore, so may not screen a block of flats - although obviously it depends on perspective and angles. They are evergreen though, so in that respect provide a better screen.
If its established, and doing a good job of blocking the view, then I wouldn't mess with it tbh. If you plant something else instead it will take years to get that big - and if the environment isn't right for it, it may never get very big.