I think I found the lens - the new 24-70mm
Incredible base sharpness and the RAW files handle very well to even more sharpening.
This is probably the best lens I've tried to date and the fact it's a zoom amazes me.
I guess I need to see what the 12-24 is like if it is announced next week.
What I HAVE discovered is that only the very newest released lenses seem to bring out the best from the 5D MK3...