In Britain's troubled boardrooms, it's too easy for a powerful minority to seize the controls

In theory, an investor needs a 50% holding plus one share to take the helm at a company. But the upheavals at three London-listed firms suggest you don't need nearly that muchHow many shares does a group of like-minded investors need to buy in a listed company before they can disregard the views of the directors and other shareholders - before they can dictate exactly how a company should be run?It is a question currently vexing fearful minority investors in three FTSE 350 firms: Kazakh mining group ENRC, easyJet and pub operator Mitchells & Butlers - the last of which reports full-year results on Tuesday.Board members at each of these firms have, in recent times, tried to show some independence of purpose, distinct from the wishes of their largest shareholders. In each case, however, they have found themselves besieged.In the case of ENRC, the combined 35% voting might of three Kazakh founders, backed by other Kazakh interests, secured the defenestration of two non-executives - Sir Ri