49.Whenever there are three or more candidates to be Mayor, each voter may indicate on the ballot paper their first and second choices for Mayor. When the votes are counted any candidate with more than half the first preference votes wins outright. However if no candidate wins an overall majority then second preference votes are taken into consideration. The two candidates with the most votes remain in the contest (and there is provision for any tie for second place). The second preference votes on the ballot papers of the eliminated candidates are then examined and any second preference votes for the remaining candidates are allocated to them. The candidate who then has the most votes is returned as Mayor.
Commentary on Sections Part I: the Greater London Authority The Mayoral Poll