I would try to get a hold of the Australian QF call centre - you can call direct +61 13 13 13 or +61 3 9834 6479. Or call after 8:00pm UK time IIRC.
It sounds as though someone in the UK call centre is being "economical" with the truth about your seating, and leaving it to checkin @ LHR to resolve. And they are precisely the wrong people to have resolve this issue for you.