Found it at last
Well this was one of the first puzzles I ever solved (back in May I think), and was pleased to find that the location is fairly close to where I work. Unfortunately that meant that I always investigated the GZ at lunchtimes when it was very busy. I combed the area in a methodical way whenever a few muggle-free seconds opened up but still couldn't find it.
Today I was in the area with reinforcements (my children CC3 and The Bobcat). Embarrassingly it was in the first place The Bobcat looked. A very cunning hide.
We celebrated with drinks at Starbucks where we also signed the log at our leisure. We returned it about quarter of an hour later (since its been six months since it was last found we were fairly confident this wasn't inconveniencing other cachers)