Mark Rogers wrote:
So an mbox pruning tool would be fine too.
To answer my own question: I have discovered archivemail ("apt-get install archivemail", always a good sign :-) which has happily deleted all old mail from the quarantine allowing dspam's web UI to finish the job.
Other suggested tools still welcome though, as it would be useful to be able to weed out based on other criteria too (eg the addressee, subject, etc).