Hi GGirl, thanks for your encouraging answer.
The office is at Road Management Center, District 12, Bao An, Shenzhen 518101 (
here, according to Google Maps) which looks to me like being in the middle of Shenzhen, though not too close to any of the trains from Hong Kong.
I don't have a HK ID currently. I'm told it matters whether the company has a HK office. To my surprise it looks like they don't.