"Martin" wrote
We have a 20 years old sycamore, also kept in a pot and about the same
height.

I'm surprised that a sycamore would have lent itself to such treatment, as
I've always found them (and Horse Chestnuts) to be thugs at heart.

If you didn't want to persevere with some of the Japanese Maples, which
are better-behaved but much more temperamental, the indigenous Field Maple
(Acer campestre) tends to keep itself compact. It has a typical acer
leaf, but looks much more refined than many of its tribe.

I've got a 1.5m, 5-year old one in a pot (found as a seedling in the wrong
part of the garden) and it looks fine.

- Tom.