Simon, can't wait to see what your buckthorn bow looks like when your done with it, I'm sure it will be much nicer than this one of mine.
I've made about 12 or 15 from this species and have broken around
6 other buckthorns while trying to get a bow out of them. The breaks were all tension failures from flaws in the wood or in some of the cases poor tillering. It is a fickle wood for sure but makes a good bow.