hi had thesame problem on a petrol version it was a solidnoid on the gear box .
was a easy job for the garage but the part was special order and quite exspensive with the gearbox oil.
I now have the diesel version lovely car just had to replace fuel pipes as these are common to fail and the car wont start due to air in the pipe
they are £90 a set from merc garage and paid £40 for fitting.