Nigel, Well the bible only tells of the deaths of two. James, who was killed by a king, and Judas, who committed suicide shortly after he betrayed Jesus. Most of the other apostle's deaths are know because of historical letters and stuff, but I've just done research and there are three apostles, John, Bartholomew and Simon, whose deaths are unknown. Thats pretty cool haha.
I found most of that info here: http://agards-bible-timeline.com/q6_apostles_die.html
I've never actually thought about that before. What made you curious about that?