Hi guys.
I have pushed an update to github which includes a solution for saving revisions when only a custom field value is changed.
To clarify the issue, WP only saves a revision if it sees a change in post_title, post_excerpt or post_content. The new code hooks in and tells WP if a metavalue has changed. If so, allow the revision to be saved!
Thanks for all your help.
Cheers
E