FTR it appears using
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" >
I know we've been in touch over email about other style-related questions. Apologies for not responding to your question here: the forums decided not to notify me of any new activity on the site, so I wasn't even aware you asked a question
Looks like you have resolved the issue yourself, selecting the latest supported DTD from Oasis. I think DocBook 4.3 is quite outdated now.
Hit me up on email if you have any more questions, Pete.
I've enabled notifications on the forum now so hopefully I won't miss any more questions.