Yeah, like Steve said, there's a fibre washer that gets sandwiched between the ends of the spigots on the tank and tap. One of them on my Jota had a persistent leak a few years ago that wasn't cured by a new washer. Closer inspection revealed that there were some corrosion pits on the end of the fuel tank spigot. A tidy-up with a fine flat file got rid of the pitting and fixed the leak.