See if you can source some Croad Langshans. Your Poultry-club may be able to help you? They are the same chicken as the Langshan in the US, and I really dont know why the Americans dont call them that. From the original stock, imported from China to England in 1872, there was developed two main lines, one taller than the type we call Croads plus the type we now call Croads. The other type was named Modern Langshan, and I only know of a handful of people that still breeds them, so they are ectremely rare nowadays. Later the Germans developed a new breed, called German Langshans. They are poor layers, and mainly an exebition-breed. Very pretty, though. When the breeder mentioned breeding lines, he ment you keep them in 2 (or more) flocks, and only cross those two lines every 6-10 years. Dont ever cross siblings, and then you will see size, fertility and laying improving. You have to be a bit hard to begin with, cull bad types, and only breed from good layers, but after a few years you will see how it works. I only keep LF (Big) White Croads, and I hardly have to cull any, as the pure Langshan is a very old breed, and they have very strong genes. If you cannot have two flocks, see if you can find somebody to breed one (or more) flock, and swap a cockeral every so often. They will be related, of course, but they are still a valuable outcross.
I will go more into detail about this in some articles I write for
http://www.thechickenwhisperer.co.uk/ so keep an eye open for that. I will have as a guess they will be published in February sometimes. There will be at least 3 arrticles about breeding, and in the one about breeding rare breeds, there will be a part where I explain how I do with my Croads.