Hi Gazza,
I have no idea what the second hose is for but to prove that there isn't a problem with the needle and seat I would plug the small hose both ends (don't cut the existing hose just use a bit of scrap hose) and plug both ends and then you can prove if the needle and seat are the problem. If the carby still leaks then the problem is the needle or seat as there is no other way it can leak
