Once all 6 collars are off, you should be looking at this:
The collars will be filthy - chuck them in a bucket of degreaser for now.
5) Removing the delivery valves
With the special Mercedes tool, unscrew delivery valve 6 - carefully.
As the valve is removed, there will be a tall slim spring resting inside the pump - take care this doesn't fall out into the engine bay.
The o-ring should be above the thread on the valve you just removed - if it isn't there, it will be stuck in the fuel pump here:
(you can just make it out, above the thread)