Thanks for your responses, but it appears to be resolved now, and disk space was not the culprit.
The problem turned out to be a Perl module dependency(s). I possibly forgot to mention that I had just upgraded from Centos 4.4 to 4.6 (sorry about that), and it appears HTML::Parser was out of date, which was causing spamassassin build and / or configuration to break.
That wasn't entirely obvious to me from the error messages and log file, but I got there in the end. Also DB_File would not build because it was missing a db.h file, and I needed to install db4-devel and BerkeleyDB (I think). A bit of trial and error there, and I still don't understand why that wasn't needed for Centos 4.4 + ISPConfig 2.2.9, but was for Centos 4.6 + ISPConfig 2.2.19.
Anyway, it looks as if I am up and running, so thanks for your support and excellent software package - much appreciated.