many groups just appear to have lived out their lives at the zoo with the odd birth but ultimately have not sustained themselves. yet the focus on breeding endangered species just doesn't seem as strong as say, chester or marwell. in a few years I don't see them having any species that they do particularly well at over any other institution. Sure, they will still hold animals as part of EEP programs but I don't think they do exceptionally well at breeding a particular species.
You're are getting a fairly accurate impression here!
When it first opened in the early 1960's, the collection of 'pet' apes and monkeys grew initially into a larger primate collection with some other traqditional 'zoo' animals added to fill in the gaps. Later this developed into a more general collection, but still with a very large primate collection.
Some of their primates do breed freely- Colobus, Spectacled Langurs etc, others less so(guenons) The Proboscis monkeys I think they did breed at least once before they died out. Generally speaking the lesser Primates are well catered for and they have developed a level of expertise with them over the years, although the housing is functional rather than at all naturalistic.
Orangutans have bred reasonably well over time although they mistakenly produced a number of hybrids some years ago- the result of two pairs being kept with only a mesh barrier between them! Gorillas they have a very POOR record-like the early chimpanzees, their original pair were badly humanised and never bred, subsequent ones have been erratic breeders and very little effort has been made to improve things. Effectively, there is currently only one breeding female out of five... Their gorillas and the enclosures they live in are one of the very worst features of Twycross, in my opinion. The Bonobo group, on the other hand, is thriving and is a good display too.
Baikal seals I remember. I think these died. Malayan Tapirs- they seem to be continuously replacing them- I don't think they have ever bred them.
Twycross is an odd mixture,some good, some bad, with an interesting collection but much of it is simply a display rather than anything else.