jon thats what I thought- just basic good mechanics practice- but jay disagreed.
I've lost track of what I disagreed with.
Jon is correct that the flex hose is secured to the chassis at the end where it connects to the rigid line from the clutch master cylinder. Since the flex hose resists twisting axially (or "coiling" as Jon puts it), can't rotate much
at the slave cylinder end. However, its modulus of elasticity for axial twisting isn't infinite, so if you really want to worry about something, while the hose certainly can't turn enough to completely back out, it could back off a few degrees and cause a leak. But the point is, this just doesn't happen.
I till like my idea of a thin nut, m12x1.0, that cinches down on the slave, like a lock nut
Well, I don't like that idea (and for what it's worth, I'm a mechanical engineer). As you tighten that lock nut on the threads, you're pulling the fitting outward. Admittedly there is only a minuscule amount of axial play in the threads, but your lock nut still decreases the sealing force between the hose end and the slave cylinder.
While some fittings are designed to use a soft, copper washer between the hose and cylinder, the Alfa clutch slave just wasn't designed that way. The surface of the slave cylinder where the hose enters isn't polished to allow a washer to create a tight seal. And using a stack of washers to create a seal is a definite no-no - lots of surfaces for a potential leak.