Sorry, I had to move on with this issue as it was going nowhere. It is impossible for a member or user to just join a Free Parent Plan when it is dependent on membership of a child Plan, as the Plan will never be visible for them if they are "still" members of the Child Plan, where the Parent Plan expired.
In the end I did an SQL update on the table to set the expiry of all Parent Plans to NULL, where they were currently set to Expire, and as far as I can tell this has resolve the issue. Cheers, D