Good point about the notation in Simpson, which I should have compared with Chappell PMOT (unfortunately I don't have his earlier work). The only real difficulty in phrasing (from my point of view) lies in the last line, where two minims certainly makes better sense on the face of it; though it actually sounds quite uncomfortable in practice. I'm thinking, though, that Dyer-Bennett's source is likely to have been Chappell, so I'll stick by my earlier comments for the time being.