I'm sorry to burst your bubble mate! I've had a few chimney fires, the last one just two years ago. Anyway, you know it's on fire as it sounds like a jet taking off, flames and sparks twelve feet out of the cowel, first thing I did was spray the fire with water, so two gallons of steam up the chimney subdued it for a few mins, I removed the grate and front and put a board up that I use to block it up during summer (stops soot fall and birds), this is where it really gets interesting, as starved of oxygen it starts consuming it from above and draws it down the chimney blowing the board out and