Hi Frances,
I have mostly used Ancestry to date, and searching on an address didn't work very well when I tried looking for Willow Terrace months ago. However, I've just tried a different site, and in 1891 4 and 5 Willow Terrace are lumped together, but there are two grocer's assistants shown - so maybe the shop owner himself lived elsewhere, with some of the staff above the shop.
In 1901, the census jumps from 3 to 7 Willow Terrace, so no more clues there.
One of the grocer's assistants in 1891 was a John James Thomas from Arbory, which is where JT Costain came from, so maybe he heard about a vacancy from him.
Thanks for your help again
MoragC