Good post WILF. Thats the problem, to many terrier getting bred for stamp/type now. Not many are breeding from tested stock.
look at the bully type thread on here. I know some of the owners, and they haven't worked dogs for a number of yrs, yet they are still breeding pups.
This, I know how its bred so I will take a chance and breed from him/her is the reason there is so many nervous scatty highly strung terriers about.
If you cant find a good working stud from the line you keep, then its got to be worker to worker.