From your description I don't think you smelled gas. I think you smelled petrol vapour.

In a sense this is a gas, but in that sense all smells are gas, so the distinction has no meaning.

If you smelled gas you wouldn't expect to see dripping, since gas isn't normally visible.

This seems to confuse Americans, but they don't usually drive Vauxhall Corsas so can hopefully be ignored in this context.

Could be clip, could be split hose, could be corrosion of the steel pipe neck. Only way to find out would be to examine it closely and maybe take it apart a bit

.Failing that, don't fill it so full.

Few pumps are self service here and I sometimes get a strong petrol smell if the yoof pump operator has overfilled it.

It seems to surge up the venting system into where the vapour trapping activated charcoal caninister, would be, except that isn't there.

This probably doesnt apply in your case

Can't reach your photo from my computer, but never mind. I have never liked the idea of brim-filling with fuel, especially in hot weather (if we get any) as it only encourages more vapourisation and smells of 'gas', also surges towards the filler when cornering. I hardly ever carry more than half a tank: a full tank of 50 litres or more means 40-50 kilos of weight, which may help rear-end roadholding but adds about half a person to overall weight.