We've tested it now hundreds of times. The anti-virus can be disabled and Office taken to online templates. Once it is done (and typically we open word, excel and ppt just to be sure we have all of the files) the AV can be enabled again with no further problems.
I understand now that on first use Office online templates downloads a series of folders and files to Office 15. It also makes changes to the registry- it is these actions that our AV blocked.