On Sraffa, see this. Garrison has a whole article on it, and Lachmann does too but I can't find it online, addressing his arguments. Murphy has a few online. I think if you want to know more you should e-mail Garrison. This also came up on the old Austrian forum, and you should read DADalton's posts on on it. He's a great guy, very smart, but he doesn't post on here unfortunately.