Keepers in many zoos routinely enter colobus or other large monkey enclosures with the monkeys present. I've worked with guenons that we went in with, but we didn't go in with our capuchins, whereas some other collections do. It varies from collection to collection, and would depend on the policy at London. I can imagine it would be difficult to 'chase in' animals that are up in trees into a small wire tunnel. Even if they are not mixed, this function of the exhibit is not an entire waste of time as the colobus are often given access at times when the gorillas are inside, and the olfactory stimulation of species rotation should not be underestimated.