OOOH, this is getting interesting. It's critical for me so I will have to ring and make my case very clear. I also think I'll be directly buying my BKK-SIN ticket from QF sales and make sure the PNRs are linked (with the date critical SIN-SYD). I might follow it up with an email just to get a written response as confirmation (that QF BKK can issue BPs for both flights (BKK-SIN and SIN-SYD) and that if I misconnect due to a late running first flight that I'm OK on the next available SIN-SYD flight even if the ticket has date expired).

Thanks number_6 for your considered replies.