By Marwa Rashad and Kate Holton LONDON (Reuters) – Russia-owned Gazprom Marketing & Trading (GM&T) narrowly averted insolvency in March by meeting a key monthly payment deadline, but concerns are mounting over whether it can remain afloat next month, industry sources told Reuters. Its collapse could plunge European gas markets into disarray. GM&T buys gas on wholesale energy markets and its British unit alone supplies fuel to more than 100,000 sites across Britain, France and the Netherlands. The company, majority-owned by the Russian state, said it has not faced sanctions over Moscow’s invasi…