[Treeve]

The restrictor with the 'flappy bit' inside is to stop evaporative emissions and forces the air purged from your tank when you fill into the charcoal canister or in the event that you forget to install the correct cap, prevents a lot of lost hydrocarbons too. It's still in all of the current emissions ADRs - in NSW you are tested to ADR 37/01 limits, but still need to comply with the rest of the current ADR (79/..)

The details for evaporative emissions are pretty in depth, and are in volume 2 of the ADRs listed on the infrastructure website. Since the mass manufacturers use this to eliminate the evaporative emissions, although you could try to argue against it, it is still a requirement to control your evaporative emissions. Not fitting one is the same as having open crankcase vents - it's just not allowed.

One doesn't cost much (try Europa spares in the UK, or car builder solutions - they've got ones for £35 or so that will do the job, and fit inside a Le Mans style filler).

Treeve