Between the elephant house and The Nest. A little building with a sign on it saying "Scales and Tails" or something similar, you couldn't enter the building, but there were three or four glass panes on one side, displaying geckos.
Toilet Block - that's what the building was. Actually, I remember a keeper I was speaking to wasn't aware there were geckos in those tanks, and he said something about tarantulas. I's completely forgotten that until you mentioned it.