the cap is not sealing. Fuel injection is closed system. Make sure nothing is on the cap gasket. Overflow is for when filling the tank. If overflow is allowing fuel to spill when closed the the gasket is failing for some reason. check to make sure you have the right cap.
I will check. There are 2 small holes in the top of the tank. Isnt one the breather hole for expansion?
As standard that hole has rubber washer and links to the cap to vent from what i can tell. With qr cap it just sits there open so i prrsumed petrol could come out of it?