It might be a good time to invest in buying a Factory CD manual. You will need the special Socket to remove the Delivery Valve and the O-ring and Copper Crush Washer.
Here are some threads:
http://www.peachparts.com/Wikka/OM602InjPumpFuelLeak
The source where I got the link from above.
http://www.peachparts.com/shopforum/showthread.php?p=1217286
The main category where I found both of the above: There are some threads for E300
http://www.peachparts.com/shopforum/showthread.php?t=227463
Some people who run VO, WVO and Bio Diesel change the Delivery Valve O-rings from the stock ones to Viton O-rings as it deteriorates the stock ones. However, I would just fix the one you have first before changing them all since you are not 100% sure a new O-ring and Crush Washer are the cure yet.
Also while the intake stuff is off I would do a check up on your glow plugs to make sure the Shop did not mess something up or as it happens sometimes even a new Glow Plug can go bad.