It doesn't really matter what the problem is with the Sumatran tigers, only that there is a problem, if in all those years, and with different pairings, only a single, hand-raised cub was produced. Re-theming the exhibit, adding more glass viewing, raising the roof, won't really give them more space from eachother or the public, or nearby exhibits.
The Servals are great but, with many kept by private breeders, and zoo offspring often hard to place, it is hard to see what conservation message they are being used to put accross....it looks to me like they have chosen an active small cat that does fairly well in captivity, possibly a tacit acknowledgement by ZSL of the limited use the current terraces have for captive breeding of cats? The zoo has plenty of room on the north bank, it would be so good to see a really overgrown, tall exhibit with plently of platforms for the tigers here.
The zoo kept a single Pygmy hippo in the Casson Pavillion from at least the early 80's onwards, in the indoor exhibit currently used by the same species each winter. I don't remember this animal ever having access to the outdoor enclosure (then used by Asian elephants), even in summer. However a guidebook from that time shows the Pygmy hippo in an outdoor pool. I don't know whether this enclosure was designed for the species when the casson pavillion was built, or whether one of the indoor elephant bays was modified later on.