After reading around, I realised I needed to add some rules and did so using RulesDuJour. Things improved, but I ended up adding too many rule sets and killing the CPU. Excess rules were removed from /etc/spamassassin and the list was pruned.
#!/bin/bash# Version 1.21
# IMPORTANT! Edit the TRUSTED_RULESETS line to choose yourRuleSets
#### End Local Settings ####
I was still irritated by the difficultly of easily managing the spam settings of individual accounts using the Plesk plugin and turned to 4psa who offter a separate spamassassin implementation (Spam Guardian) which has far superior administration on an admin/client and per-domain basis.
Now everything is setup and working well with great administration and some stats too.