Car was running a bit rough, felt like one or more cylinders not firing, so I decided to change the spark plugs.
Having now done 11 out of 12, I think I now have the method sussed. Long nosed pliers to depress the spring clip, pull forward and release; thin screwdriver to free up clip between input cable and coil; separate cable at clip; fold spring clip back out of the way; jiggle and pull out coil and connecting "cable"; remove spring clip from car (it may be possible to remove spark plugs leaving this in place if you have the right tool, I am just using the 17mm long tubular spanner that is in my original MB toolkit in the boot, so it has to be removed); unscrew spark plug...

Now comes the first difficulty. The spark plug falls out of the spanner, so how to get it out of the hole it sits in? A strong magnet may pick up the spark plug if you can get it far enough down the side of the spark plug - I couldn't. The top of the spark plug has an aluminium screw cap which, being non-ferrous, is non magnetic, so a magnet does not have enough attraction to hold onto the plug while you remove it. Finally found a length of plastic tubing the right diameter to slip over the top of the spark plug and hold it while it is pulled out. Can sometimes be a bit tricky getting it to clip onto the plug, but managed in the end. Fit new spark plug; refit spring clip; push coil unit back in place; reconnect coil unit to cable; secure with spring clip.

Bit of advice: when removing the spring clips, make very sure they do not spring out and drop down inside the engine cavity as they can be very difficult to recover if they do - yes, you guessed it.

Next difficulty. The two rearmost spark plugs cannot easily be got at with the MB spanner referred to earlier. The one on the left is possible after removing quite a few covers and ducts. On the right, it is not possible with the MB spanner. This fits quite well down the hole at the bottom of which is the spark plug, but the metal bulkhead of the car above this hole prevents the spanner from going fully down the hole at sufficient angle actually to grasp the spark plug. The spanner is about an inch too long to get down the hole under the bulkhead at the right angle. Have now given up. Will have to try to find another spanner. Bit odd that the spanner which MB supply will (after a struggle) do 11 of the 12, but not the 12th.

Now heading for my third day on what, for most cars, is a relatively simple process. Is there something I am doing wrong, or an easier way to do it?

Also, with a coil unit out of the car, I tried fitting it onto a loose spark plug - it seems to require quite a lot of pushing to ensure it is properly attached. Fitting them back into the car, the cable on the end of the coil unit pushes down the spark plug tunnel and can then be secured in place by the spring clip, but it does not seem to require anything like the force to attach it and I am not fully confident they are properly attached. How do you know if they are or not?