I see you are in Wales (UK like me).
I'm with commadore on this one...
If you want to see what you can get away with check out my one hour bow
http://www.primitivearcher.com/smf/index.php/topic,16984.0.html It was Hazel, with nasty grey patches in most of the wood, I was going to throw it away, but quickly chopped it into a working bow.
The bottom line is make your own choice weighing up the time expended vs risk.
If it's your only available wood then it seems good enough to me, I've use similar 'standing dead' wood (Maple) and once you got under the first half inch it was beautiful.
I s'pose your local climate and insect population will make a big difference too (those guys in the USA have some mean bugs, six foot wing spans on some of 'em I've heard
Del