Complex sentence: The classification of ethnicity in the United Kingdom has attracted controversy in the past: particularly at the time of the 2001 Census where the existence and nature of such a classification, which appeared on the Census form, became more widely known than general.
Split into simpler sentences: The classification of ethnicity in the United Kingdom has attracted controversy in the past. This was particularly the situation at the time of the 2001 Census, where the existence and nature of such a classification, which appeared on the Census form, became more widely known than general.