Lync for Mac 2011 cuts call on video conference

[woody]

We are using Lync on Office365 for desktop video conferencing. Lync for Mac works great 1:1 but when you invite a third person into the conversation, you lose audio and video with the message 'You have left the call' although you can still IM to the participants.

 

We have tested this on an iMac and a Macbook pro both running Mountain Lion. Any one else come across this? Can't find anything on the web.

 

Thanks