>> If a scheduled backup cant run (ie computer switched off) is it set to run as soon as it can?
Yes. It is. Please make sure you have the latest version Online Backup build 248.
>> Does the scheduled backup check a file to decide whether to upload? What are these checks? (file date/size or md5 checksum etc)
All of these are checked to determine if the file needs to be uploaded again. Even if the physical file has not been changed in the case of "MagicCode" match, the file date on server is still updated but upload is skipped.
>> Are these same checks used for real time backups?
All DriveHQ Client software (FileManager, Desktop Express, Online Backup) uses the same advanced caching, date-time comparison and MagicCode comparison. MagicCode is a proprietary version of MD5 and is generally much more secure than MD5.