Here is how I FIXED IT!!
Do whatever you can to get a complete sync i.e. same on Outlook and BB Device. Normally I could re sync when it asked me to delete a whole lot of Outlook appointments, and it would then recognise them and put them back into the BB device.
Once you've done that, do this;
In the DM sync setup for the Calendar, set the days ahead to 999 and the days past to 14. What that will do, is make the DM only look 14 days back, it won't delete past appointments, it just won't sync any past 14 days!! Problem fixed...I have been running like this without a hitch for the past week.
It seems that the BB DM doesn't like looking back to far, before it starts wanting to wipe out past appointments, this fixes it.