Ok so I think I've found my answer...
This is copied from a post in 2009 on a different forum by a chap called David Burr...
"I've used them, not quite like the one in the picture but the full-on circle hook with a similar design to the long-liners original.
In theory they are great but in practice not so. Long-liners kill all they catch, nuff said.
I experimented on the river and had four bites hooking four fish, two chub and two barbel. The chub were easy to unhook and were none the worse but the barbel were a different story. Having thick, rubbery lips the hook set deep and was extremely difficult to extract. To remove the hook you have to turn it back through the arc that it has traveled in, but with such a thick lip there was little room to turn the hook and it took much longer than any other hook I have used. I immediately abandoned any further experiments and would NOT use them on carp"