Complex sentence: Unfortunately, very little evidence remains to corroborate it and most of the witnesses are dead; nonetheless the Polish historians have found enough corroborating evidence to conclude that the existence of O'Browne, and the story of his participation in the Uprising, is very likely true.
Split into simpler sentences: Unfortunately, very little evidence remains to corroborate it and most of the witnesses are dead. Nonetheless, Polish historians have found enough corroborating evidence to conclude that the existence of O'Browne, and the story of his participation in the Uprising, is very likely true.