Well I'm an idiot - the fulltext index on page_text was missing from a default installation. It didn't break searches but probably slowed them down tremendously. Also set engine to MyISAM on page_text to avoid cryptic error messages from MySQL.
// UK lang variables
tinyMCE.addToLang('',{
insert_advhr_desc : 'Horizontale rule',
insert_advhr_width : 'Width',
insert_advhr_size : 'Height',
insert_advhr_noshade : 'No shadow'
});