15,000 JOIN RALLY FOR ULSTER PEACE
nearly 15,000 people join mass rally in Trafalgar Sq, London, to urge end to violence in N Ireland; march is largest so far in Brit Isles in 3 mos since Belfast women Betty Williams and Mairead Corrigan began peace movement; aim of march is to cement internatl support for peace movement and, for many of marchers, to show GB that numerous Roman Caths and Prots in N Ireland abhor terrorism in Ulster; leaders of march, who include Williams, Corrigan, singer Joan Baez and Jane EwartâBiggs, widow of slain Brit Amb to Ireland, express faith that movement will endure; Corrigan and Ciaran McKeown, leader in peace movement, comment; illus of Corrigan with Mrs EwartâBiggs (M)