Complex sentence: Since 1964, the Campaign for Social Justice had been collating and publicising in its journal "The Plain Truth" what it regarded as evidence of discrimination and its precursor the Homeless Citizens League had been holding marches to press for fair allocation of social housing.
Split into simpler sentences: Since 1964, the Campaign for Social Justice had been collating and publicising in its journal "The Plain Truth" what it regarded as evidence of discrimination. Its precursor, the Homeless Citizens League, had been holding marches to press for fair allocation of social housing.