Giles, I would not try getting a Sigma 12-24 online. You never know what you get with Sigma QC. Go to a store and test out every copy they have. If none of them are evenly sharp, try another store. I tried 6 before I found one I liked. Then I dropped it and it was never the same again
Matt, you're not looking hard enough. ProDive on Lockhart Rd in Wanchai sell S&S. I've heard they moved to somewhere else in Wanchai and changed names. I don't shop dive stuff in HK so I'm not updated.
Don't be too tough on me (after all I am only a clownfish )I did look up the website links but nothing led me to there.
I am aware of them so I will drop them a line.