Further update.
Upgraded XLSForm Offline to 1.11 and conversion now works correctly including track changes.
So the key point here which is not mentioned in the ODK documentation on the audit log is that as well as needing Aggregate 1.5+ you also need to convert the file using XLSForm Online (presumably as this is always up-to-date) or a version of XLSForm Offline of at least 1.11
(Not helped because XLSForm Offline doesnt tell you it is out of date)