Until a couple of years ago, Battersea Park also had Diana monkeys. I was surprised when Drusillas recieved the colchester Diana pair. Maybe they are an old/ non-breeding pair?
They were exchanged for some Lemurs(or marmosets?) I imagine they didn't breeed so Colchester traded them. Marwell also have a non-breeding pair of Dianas- the first time I saw them they were incredibly fat- now they are sleek and active- providing its the same pair!
Its interesting to speculate on the past and future of Guenons in captivity, not only in the Uk but generally. I don't think they are difficult to breed from but one of the problems has been traditionally zoos have only kept a pair of this species( of guenon) or that species and so successful breeding is often paralled by deaths- and if one of a breeding pair dies its hard to get a replacement animal so then breeding stops altogether- that would be quite true even with the existing Diana or De Brazza monkeys in UK now.
I've noticed with the Hamlyn's monkey(another of my favourites) that there is a distinct pattern in several groups, they start as a pair or trio and breed a couple of young, then one or more die and that's the end of the group,as replacements are difficult to get. The survivors get moved to another group and the original zoo e.g. London and Banham don't have them any more. Although they are breeding at Edinburgh it seems to be happening there too- just a few years ago there were FOUR adult females, now there is one. What happened? Twycross haven't bred them yet to my knowledge.
Some years ago Chessington built up a group of six Dianas- at that time it was reportedly the largest group anywhere in UK. Port Lympne have superseded that now- but the Chessington group in the meantime seems to have vanished. The Guenon species have previously been taken for granted but now its apparent they are actually thin on the ground. Its a similar pattern in European, as well as UK zoos, with mostly very small numbers in those collections that do hold them.
I think those zoos which do have viable groups(in fact these are mostly breeding pairs) of Guenons, which nearly all Diana and De Brazza, plus a few Hamlyn's and L'Hoests, nowadays) should try and consolidate their animals and build up decent-sized groups. Howletts and Port Lympne are proving its possible, though I'd like to see their groups get bigger still... Fortunately they have the space and tend to 'stockpile' whatever they breed. Twycross has the widest range of species in the Uk but only in pairs or singles and their breeding record doesn't look particularly good with this group.
You also make a good point about how, lacking any further imports, these lovely monkeys could become even rarer in captivity. Lets hope a few at least can be 'saved' as viable zoo populations.