As the petrol tank gets low on fuel i experience quite a strong smell of petrol.
I have looked/felt all over but can't find any form of a leak.
The petrol cap is not an Alfa item and when you go to refuel it is very stiff to undo and then there is a loud wooshing noise as air is let into the tank. Obviously a vacumn is being formed some where.
the vacume should be there its a pressurised system..this is a tricky one if the tank caps sealing ok(which it is)it could be a leaking seal round the rear light letting fumes in.?
best thing would be to get a propper cap first it might cure it.
Have you checked under the bonnet to make sure that the inlet and return fuel pipes are not leaking ? They're situated at the cam belt end of the engine on a rail runing along the inlet manifold from left to right.
Had exactly the same problem with my 96 V6 a couple of months ago.
I replaced the cap, with a genuine part (if you want I can give you the part no), and the green filler neck seal.
This cured the problem straight away.
You mention that you have got a non-genuine fuel cap, which in this case will mean you'll have to buy a lock too.
FYI the cap and seal cost about £12 from a main dealer.