Have they still only got a very small group(about 4) of these? At one time in the past they bred quite a few of them, but mostly males- when they sent them away and a couple of old females died it left only a very small group.
Yes, I think there were only 4 on display when I last went. On previous visits, I'd only ever noticed this big male - but their enclosure is on quite a steep hillside (like much of Colchester) so unless they all happen to be at the top near the viewing window it's difficult to see them.