I seem to have resolved my own issue. If someone else is hitting this then note the following:
backup-periodic seems to require a filename on the end of the path, the resulting file will then have this prefixed onto the final following followed by the time stamp information, etc…
backup system does not require this and will just create a file
This means that if you are using backup system to test a setup and your path is /var/tst where tst is a directory then it will work. However if you use the exact same configuration (or profile) in backup-periodic it will fail.
The documentation is the same for all 3 of the backup methods (backup system, backup log and backup-periodic) so it wasn’t clear (to me at least) that there is a difference. Given that all 3 forms can use the same store/profile setup this seems odd.
Frustrating to spend so much time on such a trivial issue…