Had a similar problem with the jag last year, admittedly a little more failure prone than the Benz but still this may help:
Is there a non-return valve in the fuel system (I assume so)? If this leaks, all the fuel could be vaporizing back to the tank on a hot start, the starting system would tend to prime the line more on a cold start, masking this problem. Anyway, this was replaced and it helped quite a bit.
The ultimate solution, found after the head gasket leaked and needed replacement, was to unstick one of the fuel injectors. The car was sometimes starting on only 5 cylinders (XJ6 uses a straight 6 for those not familiar with the model), again the starting system was better at compensating for this when the engine was cold. Continued use of fuel injector cleaner and fuel system cleaner (I use STP for the FI cleaner and Valvoline for the fuel system cleaner) have kept the problem at bay for months now, even in the hot Oklahoma summer. So for a cheap way to start, try a couple fuel tankfulls of fuel system cleaner, see if that doesn't clear up the injectors a bit and help those hot starts.