dinky - its the platic insert that sits inside the nut and extends up into the flexitube material. It cracks above the nut and before the crimped metal band on the flextube. Weird, its not overtightening that does it.
The length is the usual 9 - 12 inches. 9 is to short, used the 12" and it has a little S bend in it as a result. But not huge pressure from the bend....I'll look at the metal weaved type tomorrow or just get some copper line and bend away with some ferrel connections (I believe).