Hard Sudoku Variant

I came across this Sudoku variant (Greater than Sudoku - forget about the partial sums at the side they are from a different puzzle variant) whilst investigating a puzzle from the recent vanishingpointgame (where the key is "wh0isl0ki") viral marketing campaign. It is not as bad as it sounds in this article (about half-way down; where it is taken from) but will probably take an hour or so to solve - If you have a lot of freetime this festive period then remember it is good to think!