I really would investigate opportunities for relocating some primate species to Whipsnade. The black faced spider monkeys are a good example.
I think the Spider Monkeys, Sulawesi Macaques and even the Francois Langurs are to an extent, superfluous at ZSL. People see the African Monkeys in the Gorilla Kingdom area and I think the ones in the Cat area then go largely unnoticed.
Whipsnade have no larger monkeys at all. I think the Spider Monkeys would look good on the (squirrel monkey?) island with Beech trees which is in the central part of the Zoo.