Hi I am ok to wait till 15:00 Saturday 01/06/2013 then I will have to leave the car with my local garage as I am going away
There are many possibilities for this & the first step will be to have the car checked on a diagnostic tool such as Mercedes STAR to see if any faults are stored in the engine or transmission system.
Some models have a kickdown switch under the throttle pedal - this should be checked for correct operation to make sure it is switching. I have seen these stick or break.
Faults in the engine will also restrict kickdown - check for any fault codes causing limp mode - boost pressure faults are common due to air leaks or blocked catalytic converters or EGR faults.
The MAF sensor also plays an important part in the gearshift program - ensure this is reading correctly using a diagnostic tool.
Ensure the throttle position sensor is reading correctly using a diagnostic tool from 0-100% and adjust or replace as required.
Let me know how you get on & if you need anything else.
Best regards, Paul