Update after a while....
After some spirited driving i could heard a very bad noise coming from the rear of the car... drove slowly back home and jacked the car up to find one of the Diff bolts backed out and the other was very loose.
After some research.. it seems that i needed to get Revised diff bolts from BMW. So Ordered a set of the bolts which took a day to arrive, replaced using a DIY on the interweb, Torqued down to spec and everything is good as new.
Old vs New revised Bolt.
After some manual labour
I should probably get the underneath pressure washed next. But so far so good. Any one here experienced a similar issue with the Diff bolts ?