For decades the world's major oil producers have tried to control prices by cutting or raising supplies, which relied on all of them sticking to agreed quotas. <br /><br />But unity amongst oil-rich nations appears to be crumbling. <br /><br />Now Russia has just overtaken Saudi Arabia as the world's biggest producer and it's done it by failing to keep its promise to slow down production. <br /><br />Neave Barker reports from Moscow.
