In order to use Outlook or Outlook Web Access outside the office, the computer needs a certificate installed to validate them. The last version of the certificate concerned expired on 9 Sept 2014, and so a replacement needs to be installed. This is what to do (it's probably best to use Internet Explorer for this):
In the browser, type (N.B. case-sensitive): http://octo-oxford.org.uk/dms/clinpharmCA3.cer
When the browser asks "Open or Save", select "Open".
You should see some information on the certificate; on the first tab click the button "Install Certificate".
A wizard will start; on the opening page click "Next".
On the Certificate Store page, click the radio button for "place certificates in the following store", then click "Browse".
On the list of certificate stores, select "Trusted Root Certification Authorities", and click "OK".
Click "Next" and then "Finish"
On the verification page, click "Yes", then a couple of "OK"s and you're done.