Then I found a rather excellent little website: Neon Signs.HK, that has done a great job at collating the existing neon signs in HK. It's well worth a look. Anyway, it turned out that the sign in question is on the corner of Waterloo and Nathan (an area I used to frequently pass on the bus). Mystery solved and a great website discovered.
Here's the best view I could find on Streetview, closer views were obscured by a passing bus.