It's only a moderate dribble when the tank becomes full. It's just I'd expect the petrol station pump to shut off by itself before that point (like they have on all my other vehicles) and it doesn't. So I have no way of knowing when to stop, unless I learn how many litres to put in for a given gauge position.
Obviously the fuel must be filling up the filler neck to a point where it somehow leaks (could it be a breather hole?) without covering the pump nozzle enough to shut it off.
I generally squeeze the petrol pump lever quite hard because I expect it to shut off more reliably then, but maybe I'm doing it wrong for a Reliant!