Complex sentence: The LSSP grew out of the Youth Leagues, in which a nucleus of Marxists had developed, the leaders were mainly educated returnees from London, youth who had come into contact with the ideas of the European Left and were influenced by Harold Laski.
Split into simpler sentences: The LSSP grew out of the Youth Leagues, in which a nucleus of Marxists had developed. The leaders were mainly educated returnees from London, youth who had come into contact with the ideas of the European Left and were influenced by Harold Laski.