more of a pain so here is a far easier way of doing it. Thanks to Ferrina for the information.
What Sparkplugs Do I Need?
For the Roadster there are 2 options for standard plugs
6x NGK R LKR8A (OEM fitted).
6x NGK R LKR8AP (Platinum) - thanks Neil.
They sometimes take a bit of bending and reshaping to make them perfect though.
Smart part number A 638 581 02 67
Remove the top sparkplug HT leads. Don't pull them by the wires. Use the proper tool.
Here you can see how the HT lead remover works. It has tabs that fit under a ridge in the sparkplug connector.
With all 3 plugs pulled off of the sparkplugs, fold them back out of the way.
Use a spark plug socket on a ratchet to remove them from the engine. (anti-clockwise).
Lower Plugs - Hidden Access
The lower set are the same as
Run a pencil line around the outside edge of the number plate on the right side as you look at it.
Then remove rear number plate.
Using an electric drill and a 50mm hole cutter (I used one which is used for making stair banister post holes), carefully cut out three holes 10mm inside the pencil line as shown below.
If you want to be more accurate and you are using a 50mm hole cutter...
...measure up 35mm from the pencil line.
The hole centres from the right hand line are...
50mm, 110mm, 170mm.
You will see from the next picture that the tolerances aren't particularly tight
so don't be too worried about getting this spot on.
With an HT lead remover, a long extension piece, rachet and a universal joint, the spark
plugs can be removed with very little hassle and replaced with the panel still in situ.
The smart's own HT lead remover is rubbish. The better the quality HT lead remover
and spark plug socket with a universal joint, the easier this job will be.
Spark Plug Replacement
If you wish, apply a small amount of copper grease to the thread (read more at the end).
Change the plugs and torque to:
For 698cc cars, NGK R LKR8A (New & Used) 22.5 Nm
Do not over-tighten!
Replace the caps. After sliding them on, a gentle tap may be needed to fully seat them.
I used a long screwdriver and placed the tip on top of the plug lead ears.
A light tap with the hand is all that’s needed to settle the cap onto the plug properly.
All tight? Plug leads on? Press the leads into the slots on the engine to secure them.
Replace engine cover. These were my plugs.
The middle lower plug threads were covered in oily rubbish.
This was caused by the last service not being done properly and the plug
being loose and leaking slightly. (Full Mercedes history when the car was bought)!
This topic came to my attention during a conversation with a mechanic. Personally I have always
added a dab of copper grease to my spark plug threads ever since shearing a plug in half trying to
remove a plug from a fiesta. Fixing that sort of mess takes time and money.
The argument put forward was that lubricating the thread reduced the thread friction when
fitting meaning that the plug will turn further before reaching the correct torque figure.
Using the correct values when torquing (quite low figures) means it's unlikely to seize in place
so in theory you would not require any copper grease on the thread.
An extension to the argument was to use copper grease but to forget the torquing.
Instead, the idea is to finger tighten as tight as you can then use the ratchet to give half a turn.
This is enough to squash the fire seal ring and is often recommended by manufacturers.
There is no right or wrong answer to this, go with what you'd prefer.
Apart from the obvious time it saves, doing this mod has a few extra bonuses. You can:
inspect the turbo studs
view the upstream lambda sensor
inspect the TIK pipe
check the turbo
view the back section of the engine
Ferrina rightly points out that if the 50mm holes continued all the way along, top and bottom,
the number plate could be slightly spaced out from holed panel. This would act as a vent and would allow hot air from the engine and the exhaust to escape the engine bay.
Thanks again to Ferrina (Neil) for the information.