Got it!
I did many searches and didnt find the answer, so in the end I went for it!

This is what I found...
On the back of the clock, there is a small brass circular tab, that is soldered to a pin on the clock. The ring of this brass tab is like spot welded to the cluster metalwork.
I removed all of the screws holding the clock in place, then I put a soldering iron on the tab. The clock popped out!
I put the new clock in position, put the soldering iron on the center of the tab, and the solder melted allowing the clock body to slide back into position... I didnt even have the add solder, it worked a treat!
So there ya go!

__________________
2009 Mercedes ML320 Bluetec