We have had the fuel diltion problem for a few years now and its being a mystery. Currently though We have stopped it by turning off the fuel when the boats lying idle
What was happeing is, the fuel in the fuel tank can be higher than the engine and syphon effect takes over finding a slight gap in the fuel pump shaft and drips into the sump.
The seal needs to be done on the pump at some stage, but for the moment delighted to have arrested the problem. I wont take credit for fixing it either myself.
We had been through the mill, replacing the lift pump seal, injection pump and injectors. All which helped a little but didnt stop it.