Thanks for all your inputs, it's all exactly the sort of thing I wanted.
Couple of points raised in some of your comments; I'm aware that the 16-35 @35mm pales in comparison to a dedicated 35 prime. I'm also aware that the 50L is only superior to it's smaller siblings between 1.2 and about 2.8, beyond that, the other two are arguably better.
It appears that the general consensus seems to be not to get the 35L; my options seem to boil down to get either the 50L or the Sigma 35 and the 50 1.4...
Again thank you all for your feedback, it's really great. Keep posting and I'll keep reading, though we shall just have to wait and see what I end up deciding to get. Who knows? Not me! Not yet.
Honestly, I'd get the Sigma 35mm 1.4 + Canon 50mm 1.4.
The 50L is really only for 50mm nuts who just like that focal length. Its heavy, It's expensive, It's IQ band is limited but I wouldn't trade for anything on the market currently.