There should be no smell of fuel but there are so many minor causes that assuming Dunc didn't see damaged fuel lines it's likely to be breathers or the filler pipe. Maybe don't brim the tank before you get to the bottom of it. Apologies for sounding blase earlier, mainly based on assuming Dunc would have spotted a catastrophe in the making. Perhaps a chat with the man himself would put your mind at rest. Edit : cross posted with "lovely" Dunc.