Graham,
The cover is actually off a SLK and the 203 C-coupe. It is comprised of 2 pieces.
Usually, on the SLK, there are connectors that allow this cover to just snap on. Being that my car does not have such connectors, and is not quite a direct fit, some creative trimming is required.
I attached the two pieces together to make one piece with a few screws and some JB-Weld. I then got the hacksaw out to do some creative trimming/cutting, then finished up those areas with the Dremel. I then made some brackets to hold the cover in place.
It is much more pleasing to the eye than a bunch of wires and miscellaneous engine parts.
You can just ring up your dealer, and ask for the engine cover for the SLK. Or you can do what I did and go to the dealer to pop off the engine cover off the display SLK and jot down the part number. The two parts should come out to be around $60-80 US dollars.
Hope this helps - good luck!