Thanks for your reply Koray.
I have now installed the hotfix - I did have a look at this previously but it didn't twig that this would solve this particular problem.
I have been through the constrained delegation process and added my mailbox cluster and changed the mailbox /Exchange virtual directory to Integrated windows authentication on the cluster.
Still doesn't seem to work though?
Mind you haven't rebooted the CAS servers...
Did you need to reboot?
Thanks
John