Hi
I have similar issues. I can connect to NHS Mail using iPhone and iPad but NOT using my MacBook running Yosemite 10.10.1.
I am attempting to connect using the Exchange option within internet accounts in Yosemite. It was working OK until a couple of weeks ago. I am not sure but the recent update to Yosemite 10.10.1 may have created the problems.
Very frustrating as when the MacBook attempts to connect to NHS Mail it not only fails but also locks my NHS mail account resulting in loss of access on ALL devices, including outlook from within N3! I then have to ask my NHS Mail administrator to unlock my account and reset my password. As a result I have had to deactivate NHS Mail on my MacBook.