Thanks for your input MBenz. However, there is no problem with the E/S switch: in "E" the transmission operates on a clearly different programme, starting in second and changing up very quickly. In fact, the transmission is otherwise very good: it shifts smoothly, seems to adapt to driving style, and performs the warm up programme.
I think I'll mention it to the dealer when I get around to having the car serviced. It may be that I am unaccustomed to MB's gentle attitude. However, as I said earlier, I would expect a downshift from 5th when pressing the kickdown button at a crusing speed of 70-80mph (115-130kph), with the transmission in "S". Especially when acceleration from this speed using the crusie control causes a down shift.
It is easy to allow niggling problems to spoil ones enjoyment of a car; in fact I'm very pleased with my car and its overall quality.