The monkeys were quarantined as they had the simian herpes virus which often doesn't cause them any symptoms, but can be fatal to humans. The troop at Woburn were euthanised because of it, this could also have been the case at Longleat.
I have also been told that at Longleat you can only drive through the safari once, so therefore cant take advantage of lighting and animal activity, making it pot luck. Maybe someone could confirm if this is the case?
This months practical photograpy has a feature on shooting zoo animals in this months edition. the shots in it were taken at Whipsnade which is still one of my favourite places to shoot animals (in the photographic sense only!)