All done, 1 and 2 were the tightest, ceramic grease on the shaft and threads and tightened to 10nm , (that allows about 20% on the recommended 15nm because of the grease ). So now have three good ( sub 1 ohm ) plugs for spare.
2010 Mercedes W212 E Class E350 CDI Sport, 2006 Mercedes W169 A Class A170.
On my car they need to be sub 2 ohm...measured between the tip of the plug and the head when in situ. One reading 63 ohms was a bit of a giveaway as to which one was poorly!