We sold, moved into rented in catchment, and bought once our son had a place. I seem to recall we had to show a tenancy agreement of at least 6 months, but I don't rember if it was 6 months duration, 6 months from the time of application or what.
However, I do remember the Bucks booklet about secondary transfer being pretty comprehensive (no pun intended). It is available on line from http://www.buckscc.gov.uk/
Also, we had several queries at different stage of the process and always found the ed dept very helpful on the phone 01296 383250 . Often that is the quickest way to get FACTS.