Yeah that was the problem, works perfectly now. Thankyou

Ok, thanks for your help anyway!

Ok, so now I get this error thrown:
attempt to index field 'character' (a nil value)
by this code:

for index, plyr in pairs(game.players) do
....
plyr.character.health = plyr.character.health - 1 --This line runs fine and the players health decrease incrementally
if plyr.character.health == 0 ...

When doing that I get this error and in the docs there is no reference to a .die() method:
http://lua-api.factorio.com/0.15.3/LuaPlayer.html

I can decrease a characters health over time but once it reaches zero the character remains on zero health without dying. Is there a method for killing the character?