I think I figured it out. There's an aquatic edge to it which is very synthetic, perhaps a dollop of calone. I assume that is responsible for the headache I got both times I wore it. It's definitely responsible for my decision not to buy a bottle.
Cheapsmells.com is amazing. They've just started selling 120ml bottles of GIT for under £100!! (promptly ordered of course). They have other Creeds amazingly cheap too. It's where I do almost all of my online fragrance buys.
Anybody know how they manage to be so ridiculously cheap?