Thanks for the tip about Scopehunter. That sounds good!
I tried to use "meta.namespace-block.c++ keyword.control.c++" to style the actual namespace keyword in that scenario but it didn't work and "meta.namespace-block.c++" colours the whole thing obviously.
Actually, thinking about it, if that had worked then I wouldn't have needed it because my "keyword.control.c++" style would've done it anyway! Duh!