onbehalfof organisation sign up problem
There was a similar issue reported in https://issues.civicrm.org/jira/browse/CRM-17067 where inherited membership(s) not created due to permission check in getRelationship(). I have submitted the PR in the ticket itself and the reason why the PR is still not merged is because there are some ACL changes going in 4.6.9 and that will fix the remaining issues reported by Dave Jenkin in his last comment.