UK Car Repair: my corsa is leaking petrol, evaporation system, fuel evaporation

Question
My corsa leaks petrol at the front when the engine is on and the petrol cap is on, anit doesnt do that when the petrol cap is off,no one can find the fault, any sugestions?

Answer
Interesting; Exactly where is it leaking from? I dont know the year nor engine size of your car BUT. Modern cars have a recirculating fuel system, fuel is constantly returning to the tank.  This "agitation" causes a heavy vapour build up in the tank. In order to reduce this, a system is installed to draw this vapour into the engine when it is running, under some conditions the vapour is stored in a tank. I can only imagine that there is a problem in this system.  It may be something as simple as a vacuum hose disconnected,  Check the connections and hoses to the carbon canister/fuel evaporation system then get back to me