I had this problem on the wife's clc when her comand head unit died. Even when I removed the head unit the battery would die overnight. I ended up removing 3 fuses so she could use it until I had a new basic JVC unit installed. Put the fuses back in and the (new) battery was fine again.
As a...
W211 320 CDI (07) & W208 320 (99) and both not ULEZ compliant. I am praying to every god every night it isn't extended to the M25.
I also have a Honda XR400 motorbike which also falls short, so 3 vehicles in one foul swoop...