If I remember correctly after about thirty-six years, the hyaenas were in an enclosure that subsequently housed maned wolf and then bush dog; there were still bush dog there on my last visit in 2011 but I've not been to Dudley for about eight years so am not sure what the enclosure is used for now.
I'm sure that one of the regular Dudley visitors would be able to confirm.
According to Zootierliste, the zoo had kept the Spotted hyena(s) from 1970 all the way until 1996 when I presume they either left for another collection or died out. Either way the unused area at the back of the Chimp paddock has remained empty for years since then.