Thanks again Dave, and thanks Mwenezi as well. I've had a quick look at the Plat challenge, and have a very rough understanding on how it works. I've taken the following into consideration:

1. With the Plat challenge, we will be OW Sapphire after our SYD-LHR journey, but we’ll get there if we went with BA upon our return from LHR? Is this right? We don't actually need lounge access afforded by the status for our VIE and ARN flights as we're on the first flights out of LHR, so getting to LHR at the crack of dawn will be a feat in itself. Yes, we'll miss out on priority boarding, but from what I've been reading on the BA forum, priority boarding can be rather hit and miss anyway. Our LHR-SYD flight is in F and so we will have lounge/CCR access and all the trimmings that you might otherwise enjoy with status. So in essence, it's $500 for 2 for the privilege of priority boarding and lounge access at VIE and ARN before our flights back to LHR doesn’t seem like a particularly good investment, unless I’m missing something?

2. As mentioned before, we don’t travel often (hardly ever for work, and maybe 2-3 times for holiday, all of which is paid for – which makes our LHR trip so highly anticipated, but I digress) so to have points/miles credited across 3 different FFPs seem to be a dilution too far.

3. I understand that Qantas and BA don’t offer value for money when it comes to redemptions, but most of our points/miles come from credit card spends, which we would otherwise pay via some other mean, and so getting a J class redemption award feels like a lottery win.

Slightly different question: If I book my ORD flights on QF and provide my QFF number, will QF know that I am BA Silver and therefore afford me seat selection for free and all the other perks of being OW Sapphire or do I have to put my BAEC number in the booking?