I didn't get these errors on my home system (which has TeX Live 2007 installed), but I got them on Manuscript Central for Proc. R. Soc. B, and that's a major problem.! Incomplete \if; all text was ignored after line 7.
After two days of binary searching through possible sources of the problem, I found it. Jonathan Wainwright (the author of the document class) used a \phantom in both the \@oddhead and \@evenhead without preceding it by a \protect. The \phantom macro is fragile, which means that it can cause major problems when put in contexts that move around (like headers). So it needs to be protected.
I added \protect in front of each of the two \phantom calls, and all was well in the world.
I've posted a fixed version of the document class at:setspace needs to exist in order to work (i.e., for double spacing).