Thank you for your help Scott but I had a message from Barri who knew the answer and this is it.
The pipe actually fits to a spigot (I thought it would go on a "Y" branch) which is at the rear of the manifold on the left hand side facing the car and it points to the back of the car. The only way you can see it is with a mirror. I have take a picture of the spigot but remember it is through the mirror.
Once I had found the connection I then had to clean it off and re-connect the vacuum which was a job in its self.
This was good example of poor MB design.

Many thanks to Barri and Scott.

Regards, John