in mexico the one who invites pays, and the next time the other one invites, and it goes around, and nobody counts, and if someone is going through a hard time you just keep inviting them and nobody says anything, and i know this does not scale to a table of twelve coworkers in london, but i think the problem in this thread is not the maths, it is that you have stopped inviting each other and started going dutch, and once you are going dutch of course someone is going to notice the wine