Outlook was loading several other add-ins including Act, Sharepoint, MS Social Connector.
Disabling all but the essential ones (ie Exchange) sorted the problem.
I have not had the opportunity to go back through them all to find the culprit, but Act is top of my suspect list...
Seeing this problem both on my clients network and on a test BCM environment setup on my own network to try and diagnose the problem :
User A updates an Account or Business Contact record (eg Business Address or Notes)
User B only sees the changes to those fields by exiting...