[texhax] book facing pages are incorrect

[Nicolas Bingham]

Dear texhax,

I have a very simple document using the book class:

\documentclass{book}
\begin{document}
\mainmatter
\chapter{Chapter}
Chapter
\end{document}

However, the facing pages are incorrect! i.e. the odd-numbered pages have a large RHS margin and the even-numbered pages have a large LHS margin (which is the opposite of the expected behaviour).

How can I fix this (aside from manually swapping the odd and even page margins)?

Thanks for your time,

Nic