Given the picture you have posted and the symptoms mentioned above, I am sorry to say that you are looking at your first big repair on your 560SL.
A leak in that area is indicative of a failure of the rubber components in the lower intake system.
My advice is that you will need to rebuild the entire intake system as well as all of the plumbing. This is a big job. My advice is to do everything from the injectors to the plenum gaskets. I ended up doing this job twice because I thought my plenum gaskets were good.
A summary of the components needed:
1. Air intake bellow
2. Updated Flange for the intake bellow
3. New seal around the fuel distributor
4. Cold start air piping
5. 8x new plastic injector seal housing
6. 8x new injector seals
7. 8x new injectors
8. 8x plenum gaskets
9. 2x intake gaskets
10. Coolant (you will have to partially drain coolant from the engine)
I would check to make sure this list is complete. If you haven't done so already, I would consider doing the plugs, cap, rotor, and wires at this time as well.
To get an idea of the involvement, this gentleman has taken pictures of the process:
He also did a chain and guide replacement, but that is a different repair and not required for your issue. There is no need to remove the valve covers for this work.
Parts alone cost me ~1500USD from rmeuropean.com and autohausaz.com . Since you are in Europe, you might have to search for different parts suppliers though I don't think the costs will be that much different. Labor hours for this job will probably cost several thousand SEKs.
This job isn't hard per se, but is involved and for a first time mechanic will probably take you several days ensuring that you've done everything correctly.
As a community, we're happy to help you along the way providing advice and above all encouragement should you decide to go the do-it-yourself way.
Seeing as summer is almost over, you might wish to park the car and do this as a weekend project over the course of your long winter.
Edit: Whatever your next steps are, please heed this warning: contrary to the advice of many mechanics and well meaning folks, please do not try to compensate for this air leak by turning the air metering screw that sits next to the fuel distributor. Doing so is not a repair or a solution to this problem.
Doing so will damage down stream components of the engine leading to more costly repairs.