Complex sentence: In 2002 he started Haunch of Venison gallery with Graham Southern, a Founding Director of Christie's (London) Post-War & Contemporary Art Department, which he ran until 2001, and a Director of the Anthony d'Offay gallery.
Split into simpler sentences: In 2002 he started Haunch of Venison gallery with Graham Southern. Souther had been a Founding Director of Christie's (London) Post-War & Contemporary Art Department, which he ran until 2001, as well as a Director of the Anthony d'Offay gallery.