(I've recently located a box of 35mm photograhic slides that I'd mislaid; this box contains a number of my photos from European zoo visits in the late 1980s and early 1990s.
I've had some of these pictures digitally scanned for uploading to ZooChat. More will follow in due course.)
Is it possible that I saw this animal in the nocturnal house in 1997, Tim? I have a memory of a giant echidna in there, and I hope that I actually did see it.
As "Chlidonias" has pointed out, the long-beaked echidnas left London Zoo in 1994. (There were three of them.) How time flies; that was a quarter-of-a-century ago!
There were still short-beaked echidnas at London Zoo in 1997; maybe you're thinking of those.