Petrol tank vent

Hi!
I have this overpowering smell of fuel in the cabin of my 1989 Hijet. One "wise" mechanic told me it was the vent on the tank being blocked so proceeded to drill it out. Now it's worse. Now speaking to a friend of mine a reputable mechanic told me of a Charade he worked on with the same prob. It turned out to be the vent, but evidently the vent contained a diaphram which in this case was cracked, replacing it remedied the problem. I suppose the question I am asking is does anyone have one of these vents or know where I could get one. HELP!!!!
Peter from Tassie, Down Under.
Thank you.