Office 365 Lync - connect to external clients

Hi Folks,

We've acquired an additional office which exists on a separate O365 tenant (in case that's important).

We intend using Lync 2013 meetings to provide some off-site support but I cannot get it to work.

I can create a meeting and talk to them. I can share my desktop and initiate a video conference.

They cannot contact me or share their desktops. In cases where I have created the meeting and I am already chatting to them they can't share their desktops.

I do get an error:

"An error occurred during the screen presentation". In addition if I watch what's happening with them from one of their machines (I made the trip to seem yesterday just for this...) I also see errors 4033 & 243.

In fact the symptoms are remarkably similar to the ones outlined in this article:

http://social.technet.microsoft.com/Forums/lync/en-US/71fca51a-64ed-41c3-8079-0f290588a716/lync-2013-an-error-occured-during-the-screen-presentation

The two patches mentioned are not installed so it must be something else.

Does anyone know how I would go about fixing this?