Hi Graham, I can't entirely agree with the above statement...
I have absolutely no experience of using lines with breaking strains of 55lb (other than, maybe, loaned gear on the occasional sea fishing trip) as recommended on the first page of this thread, but a mixture of common sense and gut feeling tells me that this is a recipe for disaster with regard to tethering fish to UNSEEN snags.
I have to admit that I'm not entirely comfortable using the 12lb mono that I do on the Wye for this very reason, but as you know the thickness of the line is required for abrasion resistance. I would prefer to be using a slightly lower breaking strain to allow a possibly tethered barbel a better chance of breaking free, on the rare occasions when you are forced to pull for a break when a fish has become irretrievably snagged. Personally, if I have more than one breakage in a particular swim (if a fish was attached or not) I move on.
Having said all that, in my forty-odd years of barbel fishing experience I've only ever seen one dead tethered barbel, but obviously they would normally remain out of view. I hope I never see another, because it was a horrible sight. And I don't know if the line tethering this dead barbel was braid or mono, or the line's breaking strain. All I do know is that the barbel was tethered and dead, thus proving beyond doubt that barbel can become tethered and dead! Surely it's common sense that this is more likely to happen when using lines with absurdly high breaking strains?