a versus an. The answer is simple. If you pronounce the h, use a, if you don't pronounce it use an. I remember reading somewhere (Fowler maybe?) that the h in historic didn't use to be pronounced by the "educated" but was pronounced by the "uneducated", and that's why the myth grew up that it should be an historic regardless of the fact that the "educated" pronunciation had changed.

Bingley

Bingley