Hi, last year I was looking for the same thing but was unable to find a specimen that would stay small enough. I know the swallowtail like the prunus family as well so I decided to forgo the willow and put the cherry in for them.

I look forward to seeing responses to this thread.

SCG

Can't help Nicholas, I'm afraid. But as for SouthCountryGuy, i'm wondering whether it would be possible for you to grow willows by coppicing-basically, cutting off a tree to induce a lot of shoot growth. That keeps all the growth at a low level where you can see it and use it, and makes a large tree manageable in a small space. There was just an article published in the NABA magazine, Butterfly Gardener, which mentioned the technique as a way to grow a large tree in a smaller urban garden.
Liz