SORTED!
sbingner's question sparked a thought. I loaded over half the songs on the DMS using PHATMAC (I use a Mac with WinXP running as a VM using WMWare Fusion). I rebuilt the database and replaced the voice tags using PMM on XP.
Today I cleared everything off the DMS and loaded up all my songs using only PMM, and lo and behold everything works as it should!
The pain in the ar*e now is that PMM doesn't pick up either my iTunes playlists or and of my aac encoded songs. I could load my aac's using Phatnoise Media Manager but it won't recognise my hacked drive. Still, there are workarounds to these so I am fairly happy.
Many thanks to all for your help, suggestions and advice.
Pete