During the 1960s Kenneth Slack made ‘a unique contribution’ to the negotiations between the Presbyterian Church of England and the Congregational Church in England and Wales which led to the formation of the United Reformed Church in 1972. Recognition of this came in his nomination by the joint committee to be the first full term Moderator of the United Reformed Church, for 1973–1974. In those years he was among those who encouraged the infant United Reformed Church to engage in consultations about further ecclesiastical unions.

Although personal relations between Slack and Elsie always remained friendly, the two were to differ on the union of the Presbyterians and Congregationalists. Elsie had openly proclaimed herself in 1956, when chairman of the Congregational Union of England and Wales, an ‘ecumaniac’. She had praised the World Council of Churches and had exhorted the Free Church Federal Council to work towards its becoming a more inter-denominational forum. Above all, she had berated the different church bodies for their divisions. ‘The divided church is not whole or holy but weakened and spiritually impoverished, and Christ's purpose is delayed’, she had stressed. As she had reminded her listeners and readers on that occasion, she was the child of an Anglican father and a Congregational mother, and she was married to an Anglican parson of high church views. Her personal circumstances seemed to her, then and later, to demand her commitment to the ecumenical cause. Certainly she admitted to a passionate and personal commitment to ecumenism.