Interesting, I dont know much about this but I'm thinking maybe it's a bit easier than that (maybe). Toranas used to be slow fillers from time to time. Modern pumps are quite fast, especially the new ones. Always wanted to know how petrol bowsers work so just googled - heres an interesting link. http://auto.howstuffworks.com/gas-pump.htm
The you tube video on the first page has a reasonable explanation of how the filling nozzle switches off, its U.S. but I reckon what are the chances its different here?

So if there is not enough air comming up the filler tube (from the tank) or, perhaps, if the angle of the filler gun only fits in on an angle in such a way that its airway is constricted... I dunno, try a slower bowser? Or... pour it in with a Jerry can and funnel (BIG GRIN - slight stir). But I suppose you could consider, is the alignment of the tank correct? Can you get a decent angle on a fuel filler nozzle, what happens if you withdraw it halfway etc.

Regarding that breather pipe that goes back toward the fuel filler mouth, and the 'restrictor' perhaps a pic might help.

If you decide to go with checking all fuel system breathers etc, probably a good idea to remember that with fuel systems, whenever you take a rubber hose off, and then reconnect it, it is very easy to break a tiny peice off the inside of the hose (which you will not see - especially on an older hose or when you are working upside down and cant fully see the connection) that can then work its way into the system, e.g.; carby, or maybe even a breather, if its a big chunk? Just a thought.