I have had this problem on my last two MKII's, and I have it on my G60. It will be the fuel supply pipes that run up from under the car. Open the bonnet and on the drivers side, (your drivers side) you will see two rigid fuel pipes with flexible attached that run to the injector or carb. One supply, one return, (if its injected). Start the car up and see if these are leaking or weeping.
Rubber fuel pipes age and the clamped part will perish away. Just remove the swaged clips, cut back the perished part and reconnect the rubber pipes with small jubilee clips.
These pipes and connections are just under the cabin fan inlet, so you will get the fumes in neet fashion.