This happened to me, identical symptoms. First thing I did was to remove the boot cover, the tunnel top and the bonnet and then start the engine. It was obvious the smell was coming from the tunnel area. It turned out the leak was coming from the join between the plastic red/yellow fuel lines from the back of the car to the green, wider fuel lines that went to the engine. Because the original builder had just pushed the plastic lines inside the green ones without inserting any sort of metal insert, just tightening the clamps simply squashed the plastic pipes and did not make a proper connection. There was no sign of a physical leak, it was just a gradual buildup while the car was sitting in the garage.