Checked under the car and found the electrical plug on the transmission wet with fluid.
My local mechanic confirmed the stain on the carpet was indeed transmission fluid.
I was surprised because I had already changed this plug in 2007 and thought that was the end of that problem... not so.
The plug has been superseded several times since 2007....
722.6 owners beware.
New plug on hand to go in, along with Fuchs Titan ATF 4000 fluid and new filter.
Found a supplier of Fuchs oil (about one fifth of the price of mercedes trans fluid, yet it's the same fluid - in a different bottle).
Hopefully this will fix it.