Well an interesting turn of developments. My mate's popped round and I've tried the Sigma on his D7000 with pretty much the same result, although slightly less AF tune needed. However, when it was tuned to the point the centre point was right all of a sudden the outer ones didn't work. He's brought my old 50mm f1.8 round and that's fine on all AF points regardless of needing +18 fine tune on mine, and I've just tried my 70-200mm f2.8 and all AF points are bang on every time (AF tune set to +12).
So whilst my camera does front focus it would appear the AF points work fine once tuned (should've really compared against the 70-200mm earlier

). For some reason my camera doesn't seem to like Sigma, that's two 70-200's and two 85mm's I've tried now. I only use my sigma macro lens with manual focus so never noticed an issue with that
I'm going to send the 85mm back for an exchange and if that doesn't work I'm going to give up on Sigma I think and get the Nikon f1.8 (unless JJ would accept £600 for his f1.4

) I may try the sigma art if and when it's released though.
Yes it has been in for the flare issue.