Complex sentence: Strictly speaking, the name Wittenham Clumps, or simply The Clumps, refers to the wooded summits of these hills, which are themselves more properly referred to as the Sinodun Hills.
Split into simpler sentences: Strictly speaking, the name Wittenham Clumps, or simply The Clumps, are names for the wooded summits of these hills. The hills themselves are more properly referred to as the Sinodun Hills.