smell of gas under the car's bonnet

When I turned my heater on, I could smell gas in the car. I checked for leaks etc but none appeared. I lifted the bonnet of the car and the smell of gas was very strong over near the antifreeze unit. I know there is an evap unit there, could there be a problem with the unit?

Any assistance would be greatly appreciated. I am already waiting for a new oxygen sensor and a belt dampener to arrive at my dealership and I certainly hope I don't need anything else added to my bill!

thanks
:banghead: