Lee, I know this probably isn't exactly what you meant, but when I last went to the zoo, I took photos of that freize they have charting the history of the zoo, which contains various old photos (because I didn't want to stand there in the drizzle reading it for 20 mins). If that's of any interest to you I'm happy to put thumbnails up.
Don't they have a library and archive at the zoo ? ...... I know a while ago there was some discussion somewhere on the forum about a CD Rom you could order from there which contained all the back copies of Chester Zoo News. Think it was about a fiver and that should surely contain a good few pics ?