It's been a few days since I implemented the SpamAssassin training and I can report that the experiment was a complete success. In fact I noticed a difference after the first training batch. Spam is back to manageable levels once more and any that does creep through get fed to the trainer. Double yum.
One warning though: when I redirected my first batch of spam to the spam collecting address I went through all my folders weeding out accumulated spam and found about 500 that were worth sending on. Unfortunately sending 500 emails in a short period of time triggered Pair's spam filter and blocked me from sending any more mail! I had to offer up a grovelling apology to Pair's sympathetic mail administrator! It's a sad reflection on the state of email that we have to go through all these hoops and layers just to get to an acceptable email service.