Complex sentence: The group he led, the Paddington group, joined the Militant Group led by Denzil Dean Harber and when in 1937 a group of South African Trotskyists appeared in London, it was Haston who moved their acceptance into membership of the Militant Group.
Split into simpler sentences: The group he led, the Paddington group, joined the Militant Group led by Denzil Dean Harber and when in 1937, a group of South African Trotskyists appeared in London. It was Haston who moved their acceptance into membership of the Militant Group.