Ok, i should have clarified, it was the Air Filter plastic housing that wasn't screwed back in place correctly (and not the back corner like you would expect, it was the front corner), i could slip a finger in the gap it was that bad.
Then worse, the Air Filter element wasn't even correctly fitted to the opening, the previous mechanic had crushed one side, i seriously doubt it was sealing at all.

The Breather looked to be fitted fine, but it had oil trapped around it and gunk. I cleaned it as best i could with rag and tip of screw driver..
But i did seem to be fitted correctly.