The Tube is either cast in or pressed. I've had a couple leak and if that's what was found leaking, that's what I was suggesting removing, and refitting with the loctite (even though it's listed as a thread locking compound, it will also seal press fit fittings like these).
I can't see it being the needle and seat or it's thread.
Yup I got what you were saying, though I see no way to remove the tube.
I'll just settle with applying threadlocker blue all round it. It will soon be 24 hours since i first applied it. fingers crossed that the leak is fixed. Then i can get back to bringing my engine back to life!