Barbel like carp have huge blind spot in their field of vision, this being just past their noses (if they had them) and obviously their mouths. This leads them to be reliant on their sense of touch while feeding.
Supple braid, stiff fluoro or inbetween mono.... Which do the fish associate with danger, bearing in mind that the link is probably being pulled taut and the softer it is, the more taut it gets pulled out.
I use a good mono hooklink, Ultima Power Steel or if a slightly more supple link is desired Ultima Power Plus and if that still isn't doing the jod I'd try Ultima Power Carp.
Personally, if I encounter shy/spooky barbel, I would think more about bite regristration than anything else, if the bait enters the fishes mouth and not hooked then its a missed fish.

And even the best anti eject/bolt rig doesn't always work.
Regards line strength, anything from 6-12lb.
Ben