Car heater smells like gas !

My pick up truck is a diesel engined car, and when I put the heater on hot, after about 20 minutes of driving I get this smell coming through the vents that smells like domestic gas !!
When I turn the heat off the smell goes. Not sure if this is engine related, heater related or even oil related. The engine appears to have had a very small leak although it does not appear to have lost much. I purchased a treatment from an American company that you put in the engine that swells the seals, so hoping that will stop any small leak.
Any ideas on the smell issue ??