There are many reasons why the engine would be difficult to start, the fuel filter and fuel accumulator may be at fault or the pump may be the cause.
The problem occurs particularly after the engine has been running due to the pressure relief valve located at the back of the pump inside. It is activated by a small spring and a metal cone which in time it sticks and do not allow pressure to be relieved.
As the pump is driven by a permanently excited electric motor the pressure is reduced. After stopping, the fuel accumulator maintains pressure in the system to facilitate starting and with this pressure if the valve is stuck it develops an inner leak.
If you tap the pump firmly it releases the pressure valve and the engine starts again. When the tapping becomes frequent, is time to replace the pump.
Mercedes know this and they provide a Check Valve which to be installed by replacing the existing hollow screw to the pipe connection to the pump, effectively providing a secondary pressure relief valve and avoiding having to buy a new pump.
(Check valve $12.98, cap nut $1.49 - pic attached)
Deltacom ~ Absit Iniuria Verbis